A Marisa that instead of a hat, has a big oyster shell. May or may not be waterproof, depending. Sometimes shell marisas are referred to by "shell" or "shelly" instead of "Marisa".
It is unknown whether or not shell marisas are actually aquatic. In some stories they are perfectly capable of diving and swimming underwater, in other stories they can survive underwater but crawl on the seafloor like snails, while in some cases they can't survive underwater and 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. The shell does sometimes give them some degree of protection from predators and rapists though.
As with all yukkuris, shell marisas are very attached to their accessory which is their shell. This is why they refuse to part with their shells even though removing their shell should increase their mobility and overall survival skills.
Some parents actually consider their shell marisa children as "special" and show extra care for them. Sometimes they're even treated special by an entire clan. This ironically further reduces their life expectancy as they tend either get too pampered and end up not learning basic survival skills. The special treatment also often also cause the parent to neglect their other children which can brew into jealousy and lead to the shell marisa being killed by her own sister.
The aformentioned special treatment shelled marisas receive sometimes cause other marisas to attempt to steal their accessory (which is not difficult as shelled marisas have bad mobility). This often backfires however as the thief can't get used to the increased weight of the shell compared to their original hat.
While shell marisas are technically considered rare yukkuris, humans tend to not favor them as pets as much as other rare yukkuris. This is likely due to their worse dexterity and mobility which cause them to be a higher maintenance pet compared to other yukkuris.
There's a rarer variant of shell marisa called clam marisa which has a clam shell instead of an oyster shell