So I'm probably about a half hour away from completing my rink flood. This is my first rink and I'm noticing now that my side boards are bending over my staked supports. Next time I'll surely use some triangle brackets in addition to wooden stakes, but for now how much do I have to worry about, and is there anyway to fix it at this point?

I had the same thing happen to me in the deep end. Add more bracing as soon as you can. I propped some scrap 2x4s and 2x6s against the boards, then drilled holes toward the bottom and hammered rebar through to lock it into place. It's still a bit bowed, but the boards aren't going anywhere now. The upside of this is you'll have a few extra inches of skating surface!

UMD has a good suggestion - just keep adding support. 2x3's or 2x4's can be put at a 45-degree angle near the top of the boards down into the ground. Then drill a screw or two from the inside of the rink into the 2x3, and either pound the ground-side into the ground or drill a hole and pound in some rebar. Definitely add something though. Your boards should be perfectly vertical, ideally. If they are flexing back, at least support them. I wouldn't leave it alone.