A Marisa that instead of a hat, has a big shell. May or may not be waterproof, depending.
It is unknown whether or not shell marisas are actually aquatic. In some stories they are perfectly capable of diving and swimming underwater, while in some cases their shell are treated as nothing more than a birth defect.
On land, shell marisas are even less likely survive due to the fact that weight of their shells severely reduces their mobility. They are often unable to hunt or even take care of themselves and their life expectancy is often very low.
Some parents actually consider their shell marisa children as "special" and show extra care for them.