It is possible to get an item slide superswim going on Greatfish Island, but it is really difficult. Because there are no slopes leading into the water in Greatfish, the only known spot is to use the corner of the raft. The most common setup for this is to go into the corner and turn your back to the wall. Aim right of the pots and start item sliding backwards right as the corner of the raft you are standing on is the most visible. This will make it so that when you have enough speed, the water level will usually be at the correct height so that you can slowly go into the water on your right without splooshing. You can then superswim away.
Note: If you ever drown or savewarp in the water after performing this superswim, and you don't own Farore's Pearl, you will softlock due to KoRL trying to auto-sail to Windfall but being stuck by the island.
If you performed Outset Escape, you can skip performing this superswim by abusing savewarping flags.
Otherwise, if you don't want to perform this superswim, and if you have Din's Pearl, you can set the Wind to South to enter KoRL. He will then auto-sail away to Windfall.
Hookshot the top of the pole on the raft and leaf to land on the island. Then, go slowly near the ledge (be careful as to not fall) and hookshot the chest. This is usually a more convenient strategy than leafing to the chest from the spiral island, since it allows you to park KoRL close to the chest so you don't have to swim back to him.