These were fairly easy.
1. Roughed in a few sea creature shapes on a piece of notebook paper.
2. Lay the paper over the foam piece then go over the shape you just drew with your pen again. This will indent the foam under the paper enough that you can cut along the indent lines without needing to draw on the foam. I didn't want ink marks on my pieces so dent cutting lines were more desirable for me than pen cutting lines.
3. I used some of the scrap foam pieces to add eyes, tentacles etc. and glued them on with the hot glue gun.
Bath toys you say, why yes they are sea creatures after all but why do they have to be bath toys. They don't. BUT when the foam pieces get wet they STICK TO THE WALLS!!! Awesome!
4. Stick your kids in the bath and let the magical underwater adventures begin.
You can find an even more simple version of this craft here