On the Boating Course, you can play the fun and rewarding “Boating Course” minigame. The handy Submarine Chart is inside a Secret Cave here.
Boating Course Minigame
Talk to Loot, the sailor who runs the Boating Course, to play the “Boating Course” minigame. It costs 30 Rupees to play, but you can make up to 150 Rupees if you collect every one that you see as you sail around the course.
If you don’t cross the finish line before the 4-minute time limit expires, you can’t keep any of the Rupees you collected during the minigame. Four minutes is time enough to make two laps, so you can afford to miss a few Rupees on your first lap. To make a second lap without crossing the finish line, steer sharply to the left as you approach the banner at the end of the course.
Each time you win the minigame, 10 seconds are chopped off your time limit on the next race. For example, after you win it once, you have only 3 minutes and 50 seconds for the second attempt. On your third attempt, you have only 3 minutes and 40 seconds, and so on. It’s possible to collect all 150 Rupees in one lap (or the vast majority of them, at least), so you can build up quite a stack of Rupees by playing this minigame.
Jump over the upright barrels to collect their Rupees. Sail right over the sideways barrels. Make sharp turns by jumping and steering in the desired direction while airborne.
Blue ChuChu
Use the Deku Leaf to reach a smaller island on the other side of the Boating Course’s finish-line banner. Jump from the highest point on the main island of the Boating Course and glide to the smaller island. Defeat a Blue ChuChu there for a Blue Chu Jelly.
Submarine Chart (via Secret Cave)
Use the Deku Leaf to reach the smaller island on the other side of the Boating Course’s finish-line banner. Drop into a hole on the ground there to enter a Secret Cave.
Inside are three white, diamond-shaped switches, one on top of each elevated ledge. Activate all three to reveal the chest that contains the Submarine Chart. You’re under constant attack by Miniblins, which makes activating the switches quite a chore. Use the Boomerang and lock on to each switch after you defeat a wave of Miniblins.
Sea Hearts Chart (via Treasure Chart #32)
Find and open Treasure Chart #32 (found at the Three-Eye Reef [A4]) to pull a chest from the Great Sea near the Boating Course. The chest contains the Sea Hearts Chart, which reveals the locations of every Treasure Chart that leads to a Piece of Heart on the Great Sea.