Someone on the forum with one of the acrylic turtle ramps came up with a very clever way to keep the dock from sinking under their turtle's weight:
http://www.redearslider.com/forum/viewt ... king#60668
My 7" turtle has an acrylic dock and I've been using the magnets for a couple of years and they work like a charm.
Before getting the acrylic dock, my guy had one of the Zoo Med Turtle Docks. Even the large size was sinking under his weight. To keep it bouyent, I bought another, smaller dock and cut it into pieces and glued it to the underside of his large dock to give it bouyancy. That worked too.
By the way, a piece of cork bark makes a great basking platform. Cork is very bouyant and turtles love the natural feel. When Spot lives outside during the summer months, he basks quite happily on cork bark. Some people use it for their indoor habitats with great sucess.
It is essential that his shell be able to dry out completely, top and bottom, at frequent intervals. Since your turtle's carapace looks so good, I'd say you are on the right track in thinking that the damage to his plastron is caused by the wet conditions, perhaps exacerbated by a rough basking surface.