always ! carry at least one seaman

Since the RM Fighter Pilots and Bombers are not the best available, a Seaman will help them against others that have better planes and pilots.  I recommend using a Seaman in the 9th support slot.

But as Connan wrote, your personal attention to your active planes and pilots is more essential to winning than just good numbers on your sailor charts.

The seaman should have high fighter and bomber stats.   High fighter stats are best.

The seaman will also boost your SD number which might help you survive a
DB or TB attack.   He will also boost your repair ability somewhat to help you survive.