Well upon further review - I had the idea of maybe using a thru hull fitting - the dive platform bracket is just right slap up against it, I did see they had put some sealer in the tube. So I either find a flaring tool which Mike doesn't have or I will try your method Bush - problem is I don't have 1/2" drive sockets but I may have something that will work - I'm pretty cleaver that way. I'm doing some T Top extra feet maybe that guy has one which would be easier.
Isolate and Eliminate
-- kinda like your truck keys