I've never owned a new boat, so I don't worry about drilling holes in it. I've always mounted mine on the back of the transom. I understand some people epoxy a plate back there so they don't have to drill into the boat. Another option is to get a "thru the hull xducer". Those are glued (using special glue so you don't have any bubbles in it) inside the boat on the bottom of the hull. Personally, I would rather have mine outside for the water temp function. Did I make any sense?
Fair Winds and Following Seas
Bill H. PTC USN Ret
Chesapeake, Va