Work ABs at the Pool

Since I am swimming alone to avoid contact with other swimmers while my dislocated finger heals; I am taking the oppurtunity to try some new drills/challanges.  Wanting to do some ab work; the usual butterfly kick on back with fins is my goto drill.  Easy enough... Lets make it harder by raising my arms out of the water.  OUCH, it made my abs burn.  Here is a fun way to work your abs!  Of course, there is a youtube of it from GoSwim.  Have fun! (Watch you head at the end of the pool!)