Sailing without a wetsuit- waterproofs?

Location and boat needed.

Vision, inland, in September: I'd wear shorts and a T shirt and have a spraytop and a long sleeved T shirt in case it gets cold.

I just wear a cheap Decathlon wet suit, about £40 I think.

And if you get your own wetsuit (well worth it as you seem to becomong fairly regular) you will have the satisfaction of knowing that it is pee free

You don't need to buy a wetsuit for a course of just a few days duration.  What you do need to do is check the weather forecast beforehand, so you get an idea of how warm/cold it's likely to be.  You will nealy always feel colder on the water, and if you feel cold afloat, you will feel colder ashore if you're sitting about doing classroom stuff. 

This layers are best, so t shirt, thin fleece, another fleece, cagoule is good and better than thick sweatshirts.  Plus of course you can add/remove layers in line with how you're feeling.  Waterproof trousers are invariably not waterproof at all unless they were very expensive, but they are good at keeping off spray and more importnantly significantly reducing wind chill. Wear shorts underneath if it's going to be warm, or thin trackie bottoms oif it's cooler.

The Vision has quite a rough surface on its floor, so you might want to make sure knees and elbows are covered.  It has generous freeboard though, so unless it's really windy you shouldn't get too splashed.

If you're going to stay in the sport you can buy cheap wetsuits at all the above places, as well as lidl and aldi, though lots of the supermarkets will have stopped selling them because it's not summer anymore.  Tesco gets its wetsuits from The Wetsuit Factory who have a good online service.

Good luck, enjoy the course.