The bigger question is: would the pedometer really count water walking steps? I think the motion of water walking is different from land walking, and you might not get the same motion that would record a step on the pedometer. On land, you jiggle up and down with each step, which the pedometer detects and records a step. Your movement in the water might not be the same.
I would try this: Encase a pedometer in a plastic baggie. Clip it to a headband. Now go water walking. Count your steps across the pool. Unclip your pedometer and see how many it has recorded. If it is fairly accurate, you may want to make that your method of using a pedometer for pool walking steps. My friend Elizabeth has worked as a stage costume designer and she has an old theater tip for keeping your electronics dry. But I don't know that I'd be wearing her suggestion on my head at the pool.
When it comes to energy expenditure, you really should count your pool steps as 120 - 150 steps per minute, a little more than you would be logging if you walked 3 miles per hour. It takes more energy to walk in the pool than to walk on land.
Pedometer Step Equivalents for Other Activities