Yes you can use shells in spells, especially if you feel highly connected to the ocean. I personally use them in replacement of bottles and jewellery and find them nicer to work with due to their natural material.
There are many things you can do with shells you can make something crafty out of it,you can make candles if they are big enough you can decorate your altar with it especially if you work or worship any Gods or Goddesses that are connected to oceans.
Shells are really beautiful you can make necklaces with them sometimes I take shells and put perfume in it as a offering to my water spirits.