I wash my boat all the the time in the slip. Going to do it tomorrow. We get lots of spiders and bugs that leave there stuff behind plus all the mildew just makes the boat a mess almost weekly. I use a marine soap or deck cleaner which is environmentally friendly but you can also use dish soap like dawn in a bucket of water which is non toxic as well.
I know they use dawn to clean animals from an oil spill so it can't be that bad.
The only thing that may not be permitted is if the marina, lake or body of water has a rule against it. I know some lakes don't let you dump anything overboard and need a grey water tank.