RubyToogood
RubyTwobikes
If you can get out at any point you should be fine. Also if it's only open May to October the water temperature won't ever be hypothermia inducing. Now is a great time to start as the water will be at its warmest. You won't need a wetsuit to swim 300m in August or early September.Ok, i have read the website more closely and there are two courses - 300m and 650m. So I guess i need to practice some length swimming indoors first
Your core temperature drops a bit after a cold water swim so if it's not a warm day you may want an extra layer to put on and a hot drink but that's just for comfort really.
I had a quick look and think I see the lake you're talking about. I note that they have an adults' confidence course on Saturdays which sounds like a good idea for you. I also note that everyone in their photos seems to be wearing a wetsuit. However sometimes people swim in wetsuits because they're training for triathlons which require a wetsuit. Wetsuits are a pain and I avoid them if possible but they do mean you can get a longer swim in once the temperature gets colder.