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.
Step 2: Tie the Knot
I have included a long string of pictures to demonstrate the knot. However I am no master at this knot, it takes allot of practice. It is a good idea to try with thicker rope first. Basically you make 4 loops in the y direction, then 4 loops in the x direction on top of your previous 4, then 4 in the z dirction linking the two previous sets of loops. Then you slowly go from the end of your rope. Tuck the very end inside all the loops and slowly work around the knot tightening as you go. The following Instructables were very helpful when I was first attempting to tie a turk's head: https://www.instructables.com/id/Chluaids-10-minute-Monkey-Fist/
Cut off surplus rope, tie to your keyring using a knot of your choosing. I chose a figure of eight knot because I like its symmetry.
Step 3: Test
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 lost.