We built our first (and only) swim raft out of
untreated 1X6s and 2X6 lumber. It rotted-through in just a few years.
Painting the top might have been a mistake in longevity -- it should have been allowed to dry out thoroughly after getting wet.
The advantages were that it was lightweight (styrofoam logs), low to the water (great one-ski waterski starts -- and landings
, easy to climb onto) and easy to flip over and haul onto shore for the winter.
Winnipesaukee didn't have the duck-residue problem that we have now. (I don't remember any mallards back then -- and there's none in our old pictures).
Winnisquam may have less of the washing-machine-agitation than Winnipesaukee has today, and a wood raft may last longer over there.
(Pressure-treated -- PT -- isn't that much more money, and 2X6s are only a little more expensive than 5/4ths (which, in yellow pine, tend to be crooked and flimsy). Leave large spaces between boards for drying. I'd make it as wide as possible so it's not too tippy. Remember, too, it'll be much heavier at the end of the season.
As for myself, only one of the two styrofoam logs has survived the decades, so I started collecting 2-liter empties to float my "new" raft. (And cage them securely).
Since the state is rumored to be taxing swim rafts, I've put the project on the back burner until that's resolved.
Anyway, if you're at all handy -- as you suggest -- I'd give building it a try.
