Other than leveling them using a ship that is no higher than they are by 4 levels. (So not using the monty if any are below 97).
In future, rotate your sailors I guess. Keep the BO, Gunners and scout onboard at all times, rotate the rest of the sailor so only the lowest are onboard and follow the rule of not using a ship that over the level of your crews lowest sailor + 4.
But easy and cheap? Not that I can think of. Just level them. What level are they?