Can't say anything about the gauges, no experience, sorry. The swim platform I can perhaps help a little. Looking at it, I'd think it highly likely that its cored, either with ply, balsa or solid
wood. Any holes you make in it need to be 'potted'. An over-sized hole drilled, then filled with
epoxy and faired out over the surface. The mounting holes are then drilled through the
epoxy. Strong 'big'
backing plates will help to prevent flexing and leaking into the
core but over time it's likely that you'll still get
water intrusion.
P.