if you want to reinforce it, there are a couple things you could try. you could try zip ties and make them as tight as you possibly can. or, you could cut the top PVC pipes around a long the sides in half, cut out a little extra to fit in a T-bracket on each side, run 2 PVC pieces out of the T's and join them in the middle with another T. so basically your platform would look more like an H from a top view, and a T bracket joining them. and THEN, finally, run a piece of PVC from that last T bracket to underneath the ramp, and have it hit the ramp form underneath. feel free to even rotate pipe downwards towards the water to hit the ramp lower to get it better support.
if you want to add a little extra stability, after you run the pipe to the ramp's undearneath, put another T on the end, with maybe a couple of inches of pipe on each side to give the ramp better contact points with the pipe so it has more support and can help balance the ramp as well while it's being climbed.
let me know if that doesn't make sense and you want drawings, it's a lot simpler than i made it sound.
but if you think the ramp is solid enough feel free to completely disregard everything i just said

haha