Around Crescent Moon Island, there are barrels with pots that appear but only when you're far away from them. You're supposed to shoot the pot with the bow or boomerang so that it reveals the blue rupee and the barrel stays put. Then you can sail into it to grab it. If you try to go close to a barrel without shooting the pot they just go back into the water and unload.
But if you have enough speed (by superswimming) you can swim into the barrel before it drops back down into the water, and for some reason this gives you the rupee anyway. So you can just superswim and grab all of the barrel rupees. They reappear when you get far away from them so you can just farm them by doing laps around the island. You can actually get each rupee twice: once from the pot, and once because there's an invisible trigger around the pot that gives you the rupee, so touching both on the same frame gives you 10 rupees instead of 5.
This is considered TAS-only. For a TAS, this is actually the fastest rupee farming method in the entire game, with a rate of about 50 rupees per second, or 3000 rupees per minute if optimized. This is almost 5 times faster than Ghost Ship Farming.