R.S.
This is just a thought. Maybe he is going through a stage where he is needing some "sensory input", and right now he is motivated with water play. Since children learn from their environment, it is good to encourage this kind of play. However, flooding the bathroom is not a "mommy desired, let alone approved", way of conducting experiments with water! Have you tried to redirect him? For example, first thing in the morning, fill a kiddie pool or storage bin or basin full of water. Then take him out to the water and show him. Tell him, "This is YOUR pool, or tub(or whatever), of water to play in." If you should catch him (he seems to do well being "steathily undetected", but IF you do catch him) on his way to the bathroom, you can remind him, "This is where we go potty. Outside in your pool is where you may play with the water." If his desire and motivation to learn through water play is strong enough to constantly go back to it, giving him the opportunity to meet his sensory need in a more appropriate place just may keep the bathroom dry and give you a little more time to do something other than "mopping up"! Granted, this may mean a little more work for you in having to fill a pool or basin everyday and maybe changing wet clothes a couple times a day, but it takes much less time than mopping and cleaning the bathroom! (I can sympathize!) Also, if he really enjoyed the control of having the ability to turn the water on and off with the faucet, maybe give him the hose to "help" you water the grass or flowers. And so as to not crank up the water bill, set a timer where he can see it, and give him 10-15 (whatever you feel appropriate) minutes to play with the hose. When time is up, he still has the opportunity to play in his little water-hole! I hope this helps! I would be interested to hear if this works for you! Good luck!