Since Seamanship acts like Tactics in sea battles, does this mean Tactics by itself works on sea battles too, or only land battles? If Tactics does work in sea battles too, then I assume it's not cumulative with Seamanship, but higher values are used?

They are not cumulative, but I don't know which applies, as the only time I ever had both on a hero was when building up a worthless Pete Girly for the sixth campaign. IMHO, it doesn't matter, both of these are pointless skills.

I would guess that Tactics works in sea battles too, but I'm not entirely sure. Seamanship probably supercedes it though, with the massive bonuses given.

I'd only bother with seamanship in the Chaos campaign, and even then only later in the campaign.

Tactics can be quite strong if you carry an army around, but in the basic campaigns you don't need it by the time you could have built it up. Just grab some Immortality potions and off you go, maybe with some level 4 creatures once they are available in the map.