Seamen works in the same way as medic - when u're not losing crew, you are HAPPY that u have it - but if u will lose it even with medic aboard u'll calm yourself ("it could be more oficers killed if i hadn't this piece of crap aboard" :). It's called psychology.
you can also check how your seaman works at BB when u put him on the ship's spots (T slots), not on dry-dock slots... it will add the aiming lines, the bonus for BO :). It may give you wrong feeling that you are carying an terminator aboard wich helps you shoot and aim better, repair and move faster and else mistery things... the legends are never ended but the fact is - all ships can sunk or may be sunked.
i'll say about a seamen just few things:
For BB: No matter what u think - the Reload and Accuracy caps cannot be extended by seamen's mystical "bonus".
Also the seamen will never replace an extra ENGI. So it's just useless empty slot usage (and only psychological effect for yourself, those doctors are making money just by saying what u wanna hear:) ).
For those who may not know : REP & SD is added by ANY sailor (incl. seamen, land force, any pilots, gunners, etc.) It's just the difference on "how much" they add doe to their stats on those parameters. Add and remove some gunners from your ship - you'll see the same results as author saw on harbour with his seamen. (* SD is shown in upper-left corner).
So, the extra engi beside adding to the maximum overheat time and maximum speed is ALSO repping your ship aswell, (and he's doing this better than any seamen's seen or unseen BONUS).
Also engie adds the SD in the same way as the restorer (more than other sailors), The "endurance" skill shown when you use left click at him in harbour works as "restoring" now. That is actual if u have less than 900.
So in the end: Even if u have 14 sailors at BB6 - all slots are STRONGLY needed.
For CV (12 slots): it will not replace an slot for pilot (if u have less than 8 pilots - sell your CV6), it will also not replace sonarmen and HH gunners. (1 left slot in CV6 is better filled with an engie aswell)
For SS - poor subs have to few slots for messing up with seamen.