Warm day, warm water, low wind: Short, t-shirt, PFD
Warm day, warm water, windy: Short, t-shirt, harness, PFD, spray top
Warm day, cold water, low wind: wetsuit, PFD
Warm day, cold water, windy: wetsuit, harness, PFD

cold day: t-shirt, fleece, fleece trousers, drysuit, harness, PFD

Add some boots, gloves, hats and sunglasses if sunny.

I've done sailing in the Mediterranean sea (short t-shirt in summer, wet suit in winter) in the bay area (water is cold, so a wet suit is a must at all time, dry suit sometimes in the winter) and in the UK (drysuit most of the time, except some lucky days in the summer).

I have a now old Murray's full harness, that's the best investment I've done ever. Same model is probably available from the shop on this site. You can spend two hours on the trapeze and still be comfortable with it.

My current PFD is a simple flotation aid (I'm doing all my sailing on a lake constantly patrolled by qualified safety marshals, they are usually on a capsize less than two minutes after it happened). I would no recommend it on open or unsecured waters.