all the answers are here : http://nf-guides.com

That link does not give me answer to questions 1 and 7 at all. Bad/insufficicient/possibly out-dated answer to 2, 3, 4, 5, 6 and 8. That guide is also out-dated as there is no Soviet Union, France or Italy. No information about sonarman or planesman.

So is there more comprehensive and more up-to-date guide somewhere or can someone give me direct answer?

1. How do I know what level I am? Is my level determined by the best BO I have?

'level' is normally your bo level that is refered too.

2. Sailors with level difference of 10, receive exp debuction. Level difference compared to what? BO?
its vs ship level.... table at trainworld.us

3. Ammunition. Light, Normal, Heavy or AP ammo?

until BB i would say light only, the extra range outweighs the extra damage.

4. Sailors, is it important (and if it is, how important) to add additional sailors when the level up?

depends on what sailors, in general i would say to new players always keep them topped up, as if you do it wrong you can end up with a plie of junk if you dont top up or class on time.

5. How important learning manual aim, what everyone praises like somekind god, is compared to auto? From what I've heard, it just increases range a bit. From what I've seen, it is worse than auto (guns turning slower than in automatic) and is it even needed on DD's/CL's? And how long it takes to actually learn to use it? 6 months?

there is something called 'block shot' that auto cant do, and if you want to play BB yeah its needed, but if you want cv/ss then its up to you. also AA is really really hard with auto (almost impossible). as for time it always depends on the person, seen people learn it in under a week, but the more you use it the better with it you will be.

6. AA. If I fire AA ammo, do they blow up in the air on the point I've ordered them to land (if they would be standard ammo) or in the highest point of its trajectory?

all guns have  'golden angle' that will hit planes at max alitude, there is a list somewhere, maybe trainworld again. but see above about auto and AA

7. Support sailors. People seem to say that you should only use engineers. Are they that much better than restorers and repairers? How good Seaman is?

who says that? for BB engys and reps the nation you play will set the split between the 2. for SS sonar, planes, engy(or 2) and either a rep or rest. for Cv i like seaman, but others argue against them. medic is mostly useless at the moment.

8. Armor, does CL's and DD's need armor and if they do, how much and to where?

meh, nope. unless you want to add a little bulge

1. How do I know what level I am? Is my level determined by the best BO I have?
highest BO lvl is you ranking lvl

2. Sailors with level difference of 10, receive exp debuction. Level difference compared to what? BO?
the difference between ship lvl & sailor lvl, i believe it starts kicking in around 20 lvl's difference, rather than 10

3. Ammunition. Light, Normal, Heavy or AP ammo?
always pack a bind of AP in your 2nd ammo slot, and use heavy in blitz, light in GB

4. Sailors, is it important (and if it is, how important) to add additional sailors when the level up?

very important, because a % of your rookies promotes to experts, which you can manually convert to veterans, the more rookies you have the faster you gain experts.
experts & vets add to your ability rating, which, multiplies with your sailors abilty (say 1000 accuracy * 110 abililty from vet/experts = 1100 true accuracy ability)
so not filling them up will have you working at less than 100 % of your sailors actual value.

5. How important learning manual aim, what everyone praises like somekind god, is compared to auto? From what I've heard, it just increases range a bit. From what I've seen, it is worse than auto (guns turning slower than in automatic) and is it even needed on DD's/CL's? And how long it takes to actually learn to use it? 6 months?

on a DD/FF you can use auto, by the time you hit the CL2's you should really start working on manual aim, you want to cross your gunlines over the enemy, rather than having 2 seperate lines, which kinda make your guns only half as usefull, can take from a week to 2 months imo. 

6. AA. If I fire AA ammo, do they blow up in the air on the point I've ordered them to land (if they would be standard ammo) or in the highest point of its trajectory?
the blow up point isnt really important, the shells hit the planes, i dont think the explosion actually does any damage at all.


7. Support sailors. People seem to say that you should only use engineers. Are they that much better than restorers and repairers? How good Seaman is?
speed is key to victory, so you need a lot of engy's and a couple reps, restorers mainly only add soft damage (and a bit of rep), but since your soft damage will hit the cap by using only engies & reps you would waste a good rep/eng spot with a restorer.
seamans add a bit to all sailors, mostly used on cv's because the fighter ability doesnt cap, and most other values do cap by themself.

8. Armor, does CL's and DD's need armor and if they do, how much and to where?
a bit of belt (to half torpedo damage) and some bulge so you dont blow up if you hit 1 torpedo.
Unles your UK, then you could consider to move to a armour heavy setup (possibly by dropping the rear gun)

As for sonar and planesman......
Both are specialists.

In fact, a sub crew is a very specialized bunch of sailors.

A planesman is needed for a SS (sub).   A sub without a planesman is virtually worthless.
A planesmans job is to supply/replenish air for the sub.   The higher the lvl the quicker he can refill the air tank on a sub.   Quicker the refill...the sooner the sub can dive.   A sub on the surface is a target that EVERY ship in range will put underfire.   The Planesmans strength is Potential.  THe higher the better.  It equates directly to air replinishment.   Secondary strenghts should be repair/restore.   These are self explanitory...reduce/repair damage and keep the sub running.

Sonarman.    A sonarman is used to see "underwater".    A sonarman is needed on a sub in order to "see" other submerged subs.   If a sub can't see..it is a blind target.   Most subs in this game practice anti-sub warfare.   They try to hunt and kill each other. 

Their next use is on an ASW (anti-submarine warfare) ship.     This is a ship who's primary duty is to kill subs via depthcharge or hedghogs.  (the first drop over the side, the later are fired from the ship.  and all explode underwater.)    Secondary role is gunnery, either against surfaced subs or enemy ships.  AA is an option.     FF & DD can "see" subs without Sonarman but range is limited.   A sonarman increases their range.    Sonarman does not work on big ship such as a BB.  (Supposedly too much ship noise).   A sonarman can be blinded by ship noise...such as the DD the sonarman is aboard is traveling at High speed (Known as OH speed).   It takes time for the noise to die down so the sonarman will be blinded after the ship lowers speed in direct proportion to the time the ship is traveling at high speed.

Like the planesman the Sonarmans strength is Potential.   The higher the potential the better the sonarman will be.   Secondary strenths are repair/restore for the same reasons as the Planesman.

These sailors are very specialized.  So specialized that if you have no interest in SS driving, don't get a planesman.   If you have an interest in ASW get a sonarman.   If you don't want to go sub hunting, don't get a sonarman.   You can still go subhunting in a DD or FF without one, but it goes without saying that you will be better off with one.

Incidently, you should always add sailors when you level up.   A fully complimented sailor will add more recruits at lvl up then one that is not fully complimented.    Further, look at the percentage ability numbers of your sailor.    One that is not fully leveled up has a negative percent  or loss of ability.   Your ability is the sum total of vets, experts & rookies.
It is an efficiency rating.    Good sailors can have efficiency higher then 150%.    Thats how important it is to add sailors on LVL up.

OK, this has explained lot, thank you cambsguy, flaggelant and tulsa1 for understandable answer. So I just need to learn manual aim for AA and atleast once I reach CA's or possibly when I start advancing in CL's.

I also came up with couple likely simple questions, I've heard that there is limit for how many ships and sailors you can have (4 ships per nation and maximum 50 differend sailors). Is there really these limitations?

Also, Game Guide could do with update. Sailor part is missing all info from sonarman and planesman and it would be nice if there was list somewhere for all hotkeys. I've played now couple months and found out yesterday that there actually is zoom in this game and that F5 shows some hotkeys in battle....

try F10 for some extra buttons & settings

and the ship & sailor limit is 6 i believe, this excludes premium & event ships,
the 50 sailors can be bypassed by putting them into your HQ (for which you will have to win some extra HQ expansions during events). But you wont run into the 50 guys until you started your 3rd or 4th end tier crew.

important thing about end tier crews, bb/cv6 need a lot of crew, so you need to lvl all of them from the start, so the 6 sailors from your start crew wont do.