Re: they fell in the water!
When I couldn't find my keys after several years of boat storage and 3 house moves, I took the remote control unit off the boat and went to my Mercury dealer. They looked for a number that was on the inside of the unit. Once they found the number they were able to find keys that would work. The cost? $3.95 per key. I hope your dealer is as honorable as mine and won't try to overcharge you.
I believe the number is on the ignition part that you put the key in. You can probably take the unit off the boat (you don't have to take off the cables, just remove the bolts that hold it to the side of the boat) and remove the ignition part yourself and take that to the dealer. I had to replace my cables anyway because they had been cut when the lowlife stole my previous motor. That's why I took my unit to the dealer.