Re: How many of you do this?
When I trailer, I take the key out of the ignition and drop them in the console compartment, so they won't blow/bounce out on the ride. Extra key is in the tool box on board, and another is in the car. If you drop your key overboard when you are away from shore you will wish you had the spare on board. And while key floats never fail, the string or chain or ring between the key and the float will. BTDT.
At a ramp: it is seriously unlikely that anyone is going to take off with your boat, unless you are at a seriously ghetto launch (and we have some around here!). I wouldn't think twice about leaving the keys in, amd it shjold be part of the launch check list.
Even in slips and moorings, the main reason you don't leave your key in the ignition or otherwise visible is not so much for thieves who would steal the boat as for joyriders who would "borrow" it. Keep an honest man honest. I often leave mine in the marsh and take off walking, so i won't carry the key for risk of droppnig it miles away, but I might not leave it dangling, either.
Another reason to pull the key out overnight is to verify that the ignition is off and so you don't drain your battery. That's why I pull it in remote locations, but leave it aboard.
But think twice about leaving keys in when there are little ones about.