Seashells are used for a special way of getting upgrades and increase your attack strength, speed, defense and other things that can be used by these seashells. you find them within the waters in rocky like stones, some even have crabs. The ones on the surface are rare to come across if you get a few sell them to Sculley. He'll give you something very nice to add to your home once you've gather all the seashells you can hold.