The canon can be tricky for new players. You set it up, load it with your coconuts, and R to use it. Hold shift. S to aim higher. Hold the LMB for a few seconds to get it charged up and a further shot. Wait until it's about halfway charged then let go of the LMB. It's more or less the same on boats. It's good practice if you ever want to fight Vell. This event is more than a week. Even getting 3 a day you will still have enough to get the swimsuit in the end. The reason we did not get the swimsuit right away is because it's not just a cosmetic thing. It goes in the underwear slot of costume. Underwear is usually a Pearl shop only item. Having a full 8 piece costume set has benefits like 10% reduction on durability depletion and death penalty.