Seaman isn't your problem.

You need to ensure your Gunners have decent ability in their ACC/RLD areas, specifically ACC.

Secondly, you need to beef up on Vets and experts and run full staff levels. 

and lastly, you need to devote time + games = EXP = Levels = more vets and experts = better aim.

essentially, low level sailors suck.

thank you for this short, yet clear explanation

That was only part of the answer on gunners, now for the rest:

Depending on base stats and bve status and gunner path ( if US or UK gunner) your gunners will reach the accuracy cap anywhere from somewhere in the 60's till somewhere in the 80's.
Once you reach the cap, no extra amount of veterans or experts or boosting effect from a seaman will improve your spread. But the accuracy cap is level based, so by levelling your gunners you can improve your spread.

As for spotting your shells, yes a seaman will improve it, but it's far more effective to put experts/veterans or even a boost on a BO, which shouldn't be all that hard to get with the on going event if you are really worried about. But this is a problem that goes away rather fast. BVE BO's typically have to have recruits removed around lvl 90 to stop the guidelines from becoming longer than the gun range..

Lastly, even at lvl 120 the boosting effect from a seaman is rather limited. For most stats it doesn't go much higher than 5%. So I'd think long and hard before using a seaman on a BB.
That spot is imo better of being used by an engineer or a repairer.

indeed i might be a nub in bb, but i have found seamen to be usefull, however i also have the eng, and repair to fill spots if i decide to change my set-up.   Granted my gunners are way overveted almost 300 now, but i do see a diffinence with the spead when i take the seamen off, also takes me roughly 3 sec longer to reload with out them.  when they are on it takes me 6 secs to reload on my mn bb5.