Theoretically it's useless because not many test has been proven or those that were made, the results were the same as having no seaman. It's mostly based of how you manage (micro/macro) control your planes. Of course the stats of the pilot has to be taken into account too. Or at least not until I left the game some years ago. Not sure about now if they've patch anything new. 

well...i put together a hindenburg KM crew with 6 FPs, 2 DBs, an engineer, and a seaman in the supp slots(max 8 pilots allowed).  in the gun slots i put a gunner in the aft where the most guns are(PHH), feeling this would be adequate if i got into a snit(3 turrets per side).  
no guns in fore slots and placed a 2nd seaman there.  it made me feel better.  
 
the speed was ok for a run or desperate charge, 21 PHH launchers per side, and i STILL believe F or B based seaman have to contribute something(and for every class), if only a tiny smidgeon of a boost...and they are stackable ...and they work in T slots also.                

Medics are supposed to prevent pilot death(recruits, experts, vets), but i have only actually ever seen low level scout pilots experience these losses.  my only medic is a token KM one at level 70+ and he will prolly stay right there.

The effect of a seaman is marginal, I'd only consider bringing one if you plan on bringing heavy vetted FP. (adding a very small % of ability isnt very usefull to use a crewslot for if you didnt bother raising ability with all other options)

Since the effect of 1 is allready marginale I Never advice a 2nd one, because there are more important things you can bring in your support slot.
however a medic isnt one of them, crewloss isnt that bad usually, and when you get to 120 what will be his purpose/contribution in battle?

Id go for a gunner, or a sonarman (sonarman should go to the support row and the seaman into an unused gunslot)


@tech;  on the hindy you can run an engy in the Bo slot as well, comes in very handy

A higher level Seaman can maybe boost a Poorly vetted or low level flight crew. On most CV's however you are better off with a Repair and gunner or something to level.
Medics were turned off a while ago for some reason.

seaman has stacking penalties.

besides, i use sonar + 2 support sailors to use HH with the help of the rep stats