Hi there! Thanks for playing!
The difficulty has been a point of contention amongst the dev team haha. Mostly in relation to the amount of crates needed to level the cannons; adding to this you can actually shoot the boxes with the cannons to collect them (although we didn't telegraph this very well) but we do find it speeds up their collection a lot! Ironically we nerfed the first cannon somewhat to make it feel weaker so that the next tier of cannon felt more like a substantial upgrade, once you get there though we think that fighting back and using the cannons feels much more intuitive.
The movement itself is meant to feel somewhat floaty, what with being hurled around a whirlpool. We had nerfed the speed a lot with the sails down to give people a safety net for recovering and also lowered the amount people get dragged into the whirlpool's center. But overall crashing into rocks is a consequence of being too focused on one aspect of the ship for too long instead of balancing the tasks. Admittedly there is a learning curve to it, and the short play session times jams encourage don't really lend themselves to learning the intricacies of the sailing.
Glad to hear you think it's well put together though and liked the idea! Personally I'd love to expand upon it with some further tweaks to the cameras, controls and balancing. Because we have had a lot of feedback about those areas specifically haha.
Out of curiosity, how far through the fight did you manage to get?