I just installed a Humminbird unit on my Revo. Bought the exterior mount transom type transducer and am using it inside the yak. They sell pucks but they aren't necessary as long as you mount the transducer pointed in the right direction.
The external units mounted internally are what most everybody I know use.
I was told that if you get bubbles in your adhesive, it can adversely affect the performance of the transducer and give poor readings on your FF so I set up a water bath made from some closed cell foam and clamp the transducer to the mast post. I get great readings:
