I want to install it on the front of a pontoon boat. If I attach it to the underneath of the floor, will it work correctly? There should be a air gap of about 18". I know some work "through the floor", but I think there was water touching the bottom. Another option is a trolling motor mount, but the under floor would be so easy if it would work.

I would think drilling holes and attaching to the pontoons would be a big no no.