The XL sharks are to be unlocked once you max level up any L (Large) shark.
High health and a big mouth~ in-game description.
Length: 23 feet ( seven meters)
The Megamouth is by far, the worst Shark in its category. It has really good health but low stats is comparatively easier to buy than the Great Hammerhead or the Tiger Shark. It also has the same diet as the two.
In real life megamouths are harmless to anything bigger than a shrimp.