If it is an under water obstacle course you are after, we have swim through rings that sink and sometimes float suspended that we have a lot of fun with. There's also a water slalom hoop set both are available on Amazon. As for some sort of box/platform, how here's a few ideas; You could MacGyver something that may be suitable by zip tying plastic milk crates (or something similar) together and putting weights in them to keep them on the bottom? You can buy the crates in colors to suit your taste and they stack to add height. Zip ties come in all sorts of colors, lengths, and strengths as well. I'm thinking of something Lego-like that you may be able to tailor to suit your needs. (Plastic and modular) I'd avoid anything metal as it may rust. You could weight it down with the cheap plastic coated concrete barbel weights, the rubber bricks in the above post, or something similar that would be pool friendly. I got my kids a modular skate ramp that is heavy plastic so they can configure it to suit their desires without my assistance. The platforms (Box) on one of those may even be something that may suit your needs. We have the "Land Wave" brand. It is on Amazon if you want to give it a look. In all cases, I'd consider the potential to damage the pool (liner tear/puncture, concrete finish, etc), anything that may "leak" into the water and throw off the balance or induce undue hazards, and most importantly, safety of use.
Given the lack of replies I'm guessing there's not much out there commercially available that is specifically designed for what you want, so it may take some ingenuity to find your solution. Hopefully these ideas will help light that fire.
