They have a stealth feature that nothing will attack you while riding one. If you attack something, say, a megalodon, the meg will attack once, then leave you alone while you’re killing it. But if you dismount, then get back on, the stealth is disabled. WARNING! NEVER ATTACK CNIDARIA! If you do, you will die and lose your ichthy.
This thing has no equal on speed in the water, just be very careful after investing points into it since the lag can cause some things to appear right when you are on top of them. Also they are very squishy so a pack of jelly fish will usually spell the end for it if you get caught.
How to exploit it: creatures (in mobile version) can’t see you on its back and you will not attack you. You can shoot arrows at megaladons and live. Easy as that. New best way to game a megaladon is the Ichthyosaurus. (Trust me I tried and it worked!)