Love these two books about the ocean. They are both very simple with only a little bit of text on each page. I especially love Ocean Babies with illustrations by Hiroe Nakata. I am a big fan of her artwork and it is simply charming in this National Geographic for kids book.
Rub a Dub Sub is an exiting trip down to the bottom of an imaginary ocean, with fun rhymes and repetition.
Both would be fun if you're doing an ocean theme for home-preschool or simply to eplore the concept of the ocean with your preschooler.