The Mola Mola is an actual fish but more commonly referred to as the Ocean Sunfish. It is currently the heaviest known bony fish in the world. Due to their size they are rarely attacked but are vulnerable to shark and orca attacks as well as humans fishing them, sometimes accidently. They are considered a delicacy in several asian countries, including Japan, but are banned from being fished in the European Union.
Interestingly they sometimes float sideways in a behavior described as horizontal basking. This may be where the legend of these fish rescuing shipwrecked sailors as one can imagine clinging to the side or even climbing atop the massive sideways fish.