re: Transducer mount-what kind of adhesive

5200 is fine and the right stuff. You don't really need to put anything on the mounting plate. The purpose of the 5200 is to lock the screws in and to keep water from getting into the transom core. Not to glue the plate on. The screws will hold the plate. You may even cause damage to the gelcoat if you ever have to remove the plate. Just shoot 5200 into the screw holes and screw the mount down. A little 5200 will ooze out of the holes and that's enough to create a good seal and keep water from getting into the core of the transom.

One tip. When you are shooting the 5200 into the screw holes, use a toothpick to work as much of the adhesive into the hole as possible. Otherwise the air that is trapped in the hole will keep the 5200 from going to the bottom of the hole and you won't actually have as much in there as it appears.