A Seaman does boost fighters and bombers, but whether it's significantly depends on your interpretation.

A lvl 120 seaman with 12 ability growth ( 12 base unboosted stat or 10 base +boost) in a stat will at lvl 120 give another sailors ability about a 4% ability boost. 
A lvl 120 seaman with 14 ability growth ( 12 base stat+boost or 10 base + nation bonus + boost) in a stat will at lvl 120 give another sailors ability a 5% ability boost.
With a possible nation bonus and/or elite base the boosting effect should be higher.

A few test here: http://www.opennavyfield.com/smf/index.php?topic=839.0

Fighter has ablility cap of 4400k ablility show *(vets*4+experts)= ture ability

when u dont get the cap ,like KM +12 fighter class on time with 200 vets full experts

it dosent get the cap

take a seaman

see the seaman's  ture ability is the ablility that add on ur fighters ture ability

if its total larger than 4400 k, conratulation u got the cap

I did testing of seaman along with some other players.   We found that seaman do impact other sailors abilites on a percentage basis similar to the discussion link in this thread.  It is strictly a percentage boost, not an add of any sort.  

We also found that seaman became somewhat specalized based on their stats.  In other words, a seaman with high repair/restore stats impacted Soft Defense more then one with lower repair/restore stats.   Same with torpedo and gunnery stats.  In testing, a seaman with higher engineering stats helped with OH speed whereas one with lower engineering stats did not impact OH at all.

Because its very hard to test seamans "boost ability" on pilots, we concluded that a seaman for a CV should have high Fighter/Bomber/Aircraft stats based on the performance boost of seaman on other abilities.

This means that you should consider rolling the dice for the desired stat when you first make this sailor.