Teleport or walk to Portoga if you're not already there, then leave through the west exit. Take to the boat to the shoreline, then use the shell flute to call the dolphin. Equip the rabbit boots, get on the dolphin and move him so that he's facing up at the shoreline. Now ram into the shore and jump as you hit it. Make sure you jump before the dolphin says, "Take care!". If you timed it just right, after you make the dolphin's dialogue box disappear, you'll land on the water and keep floating over it. The game now acts as though you are using the Fly spell, even if you don't have it. Your magic will start counting down (normally, jumping with the rabbit boots does not use magic.) You can float over the water as much as you want because even when your magic reaches zero, you'll still continue to float. You can even go up the waterfalls. This might take multiple tries as the timing and position of your jump are critical.
Taken with permission from Flying Omelette.