For the record, Maui is perfectly capable of swimming in human form (and seems both reasonably buoyant and pretty confident of his chances of swimming at least a mile) but it seems unlikely that even a demi-god could swim the hundreds of miles required to get to the next nearest island. He may simply have been using hyperbole. With his vast muscle-mass, he most certainly can't float as well as a boat.
Interestingly we see (in the concept art below) that he's probably tried hundreds of schemes to escape his island, sytmied by an unforgiving sea and the sheer remoteness of his solitude.
From a storytelling perspective, a demi-god without an Achilles heel is just plain boring. He needs to have a weakness in order for there to be genuine risk. Maui's failing is that he can drown and doesn't have the kind of inexhaustible energy needed to swim hundreds of miles.