Playing the cannon mini-game on Spectacle Island can be very frustrating, as well as costly, if you fail to properly complete the mini-game the first time. (Each try costing 50 Rupees.) Instead of having to continuously readjust the cannon's angle and position to hit the barrels, get in the King of Red Lions and sail a ways in front of the mini-game post. You should place the KoRL within the range of the cannon, with the closer the better (the minimum cannon angle being 15 degrees), as you will just barely be able to swim to the island before your air meter gives out. Now when you are playing the game, aim the cannon at the KoRL, which will take hits same as the barrels, only for an infinite number of times.
There are several to set this up. If you came from another island via boat, you can get storage before sailing over to Spectacle Island, put the boat in the correct spot then superswim the rest of the way. You can also simply swim normally to the island but it's pretty slow as you have barely enough air. Finally you can use the following curiosity: