Main differences between pool and lake:
1) It's colder. Practise so it doesn't leave you surprised. It's not bad-colder, but it does take a little getting used to. If you surf or do other open watersports, this isn't much of an issue.
2) Sighting. Every half-dozen or so strokes, look ahead out of the water to ensure you're going in the right direction. I'm fantastically bad at this (due to lack of drill, lazy andy!) and I've pulled a 180 in a river and not noticed until I got back to the start. While this gives the safety boat guys a good laugh, it scores no points in the race. Zig-zagging really slows you down.
3) Visibility. Much open water is as clear as a latte. It can unsettle some people. Again, practice makes that just fine.
4) Other swimmers right up against ya. If you're happy to mix it up, get stuck in. If you're fast, start and the front and flee the frothing melee; if you're slow, let the bulk of the wave start before picking your pace. Our tri club do trampling drills where they set the slow guys off first and the fast guys afterwards. Cruel but fun, sorta. I'm a slow guy.
5) Currents. The beach may have rips one way or another. This is a local knowledge thing as rips change with the tides; ask a local
The site makes it look like a beautiful race! Have fun!