Cannon vs. demolition.

But remember, most Captains keep Battleship at level 1 for a long time, to avoid the long cooldown. So it gets destroyed quite easily in bosses

And even if you bothered demounting and remounting to keep it alive, you miss out on spamming Battle Cannon

Uh what? Have you even seen a Korean Captain build? Most of them max Battleship right after maxing Cannon. And getting off & back on the Battleship takes a lot less time than the cooldown would..

I agree with Arroz on one aspect though, these two skills shouldn't even really be compared. But Battleship is definitely a worthwhile bossing skill. Cannon is the most powerful attack a Captain has, of course they'll be using Battleship at bosses.