Arrr, here be draguhh... submarines.
Like most other reviewers, I took ages (relatively) to get past that second boss. It's harder to do than you'd expect on that point in the game (the final boss on the other hand is a bit easy to beat perhaps, especially before you know what, but very nicely done). The upgrade system works well, forcing the player to make some interesting decisions. All in all I really like this game, it's something different for a change. Keep the good work up and the rum flowing.
And for other players that can't get past that second boss: upgrade your speed. Not enough money to do that and pick up a decent gun? Then start by buyig a loot upgrade and sell that when you're done with it.