I got fed up with transparent PVC bags that show the world what you take with you when swimming (and mine still had the Tweenies on - the boys wouldn't use it anymore!). This bag comes out of less than 1/2m (or 1/2 yard) of oilcloth/PVC coated cotton.
On the inside there is a pocket for your watch and other small items, a keyring loop so your keys don't fall out and on the outside I have put a small transparent pocket for my pool membership card so I don't forget it.
Step 1: What you need
A piece of see through PVC a bit bigger than a credit card
1" (2.5cm) piece of velcro (sew-in is best for this)
6" (15cm) piece of 1" (25mm) wide twill tape
2 yards or 2m of cord (cut into 2 equal pieces)
marking pen or pencil
6 eyelets (optional)