You can not superswim normally with camera lock to Greatfish Island, because the cutscene would softlock with camera lock. You also cannot enter the cutscene with double storage as it doesn't skip the cutscene and still softlocks. However you can stil superswim to Greatfish early if you cancel the camera lock before the cutscene starts. To do so, try to build up a lot of speed and get on the island before it fully loads. Then pull out the sail as fast as possible (or a letter or anything that cancels camera lock) to cancel camera lock before the cutscene starts. This makes watching the greatfish cutscene before beating FW a possibility.
Note that before beating FF1, drowning anywhere will put you in the middle of the ocean in the quadrant you drowned in. Thus before beating FF1 another way to superswim to Greatfish early is to drown anywhere in the Greatfish quadrant outside of the cutscene trigger. After drowning you can just swim to the cutscene.
Get double storage. Superswim to Greatfish Island and drown. By activating the cutscene with storage while you're in a drowning state endless night will activate, which is necessary for both the pirate ship to appear on windfall and Nayru's Pearl to be obtainable, and it skips the cutscene.
This trick should not be used if you have to salvage the triforce piece on Greatfish Island, or if you need to go back to Greatfish in any other way. This is because going back to Greatfish will cause an unavoidable softlock.
Hookshot the pole and pull out theleaf to get to an upper ledge. From that ledge you can hookshot to the chest. This makes it so you can get to the heart piece without having to leaf from where the korok is on top of the spiral.
Superswim with double storage to Greatfish and pull out the wind waker to cancel camera lock before the cutscene starts. This skips KoRL talking and the fadeout to endless night. This is faster than the standard Greatfish superswim but the wind waker pull is frame perfect, making this TAS-only.