i rather get another engy with same base stat and level of the other engy

Your better off with a second engi. The seaman doesnt boost really sailor ability all that much.

Use a engy or rep.

If you want to do something useful, you could run a test and see if a seaman helps or not on a SS.

For SS5, I put one on. As both Sonarman and Planesman are uncapped, I believe they boost those abilities.

However, I haven't done any testing on the test server regarding them, so this could all be placebo.