Here's a bracket I made myself about 1976 that worked real well.
A good friend of mine had just bought a new Seafari after taking a ride in mine, and he wanted a swim platform too. We got 8' of 1" SS tubing, a 8'x1"x10" piece of teak (for $35!
), used a conduit bending tool to bend it into a U shape, and then used his Shopsmith to cut the teak into 2" planks and drill holes in the tubing. The plank mounting screws were recessed and covered with teak plugs. We used SS bimini top fittings to attach it to transom so it could be folded up. It was very light (an important consideration if you have a 20 with a heavy motor!), and easily removable if you didn't want it in the way when fishing. If you're into serious diving, mount it right at the waterline so it's awash when lowered, so easy to get on without using a ladder.
If you're a REAL serious diver, here's a full width platform that will also serve as your back porch!
