Its not a good idea to drop your keys straight into the sea. Test in your sink first. Then be safe in the knowledge that they will get wet, but not lo...
I live near the sea. I sail and fish and have more than once dropped my keys into the water. I find self inflating keychains, while successful, to be very expensive. Another solution is a cork from a champagne bottle or cork sphere attached to the keys but these are often unattractive. In this Instructable we are going to make use of a knot called a "Monkeys fist". It was used in days of old to store long lengths of surplus rope on ships and rebellious sailors would slip lead weights into the middle of this knot to make a weapon. By using floating rope to tie the knot, a large amount of floating material can be attached to your keys, thats a good thing!
Step 1: Get some rope.
For a pocket sized keychain your going to need rope of diameter 4mm or less. The rope must be floating rope. Choose a colour you like, or whatever you have found/have lying around/recycled. Get a sizable length. It will surprise you how much rope is in this knot and about a meter is a sensible length. I found this black and red rope on the beach. It pays to shop around.