That is the idea behind the Aqua Notes notepad
Summary from their website:
AquaNotes® is a completely waterproof notepad that allows you to write down all your great ideas while in the shower. Water simply beads up and rolls off its waterproof paper. AquaNotes is so durable, you can even write underwater. AquaNotes is recyclable and environmentally friendly. The notepad is printed with soy-based ink. Made in the USA.
Each 3″ x 5″ pad has 40 sheets of paper and each sheet is perforated for easy removal. AquaNotes® easily and quickly attaches to any smooth surface with two suction cups that are secured with a stem-based cap.
Each pencil is made from 100% natural cedar that is water resistant. Secure the pencils with a suction cup pencil holder that provides convenient access whenever the need arises to write down an idea while you are in the shower.
Check ’em out here. You can get a back of 40 sheets for the low, low price of $9.48.