The more important question would be "Is one seaman worth using?"

Well i do believe its all depend where you use it.

for example;

I find one usefull in CV mode room, i put him in the gunner slot, on support slot its all about fighters, dive bombers and engineer. Since my ftrs are like 89s compared to enemys 100's ftrs

I dont really use gunners there against cv or use AA since my team cvs will be using fighters just above us. 

I dont know if seaman FTR stacks or not

Seaman give such a small boost, something like 5% at the most, and with another the gain is even more minimal.

From what ive read the more seaman you use the smaller their boost ability is. like the first will give 5%  and the 2nd seaman will give 3% as an example.

It makes a small difference but i personally think if you can manage your planes well your better off with an engineer or rep or even a scout if you havent exceeded the max number of pilots.

or you could put the seaman in the gun slots

More important, will his 5% boost ever be usefull?

In testing, a group of us could not really offer proof that a seaman helped fighter stats.   Too many variables involved.     However, we did find that there is a non-linear boost to other abilities.
For example...the addition of a seaman boosted soft defense.  One could individually place each sailor on board and record each individuals contribution to soft Defense.   Adding a seaman increased the Soft Defense greater then the sum of each individual sailor.  There was positivily, without doubt, a boost.

In some cases, we noted an increase in OH speed by 1 knot and a slight increase in duration.

Since we saw boosts in different attributes, we believed that there was a boost in all the aircraft abilities, fighter and bomber.

We also found that the seamans boost was related to their abilities, i.e. a high restore ability of a seaman boosted soft defense more then a much lower ability.   So we concluded that a seaman with high aircraft abilities likely boosted aircraft performance more then a seaman with lower aircraft abilities.

Is it worth adding a seaman on a CV?  For sure it will help particularly with lower lvl pilots but the boost might be nerfed some as the pilot lvl gets higher.   We couldn't prove it, but that was the general opinion of the testers.  (There may be a cap of some sort on seaman boost abilities.)

As for stacking seamen, we did not test that as we were only trying to determine how a seaman "worked".

The testing convinced me to put 1 seaman on my CV.