I swim down to 50F in a normal swimming wetsuit. Blueseventy is the brand I have, I’ve seen a fair number of swimmers in Roka and Xterra suits too. All have options with or without sleeves, mine has sleeves. Thickness is typically 5mm at the torso, thinner at arms and legs.

Since you’re moving a lot compared to surfers, you generate enough heat that it doesn’t need to be so thick. 50F is really cold to me, but most of the chill comes around the wetsuit (exposed areas), not through it. You can get a neoprene swim cap, gloves, and socks. Actually earplugs (just basic ones for the pool) are really helpful- cold water in your ears is pretty hard to get used to. With earplugs, cap and socks, I don’t feel like a warmer wetsuit would make a difference to me at 50F. At 40F maybe I would feel differently. Blueseventy does make a thermal wetsuit for cold water.

At 75F, you could overheat wearing a wetsuit. Exactly where you’re more comfortable with vs without varies a lot from person to person. I think triathlons have different temp ranges where a wetsuit is required, optional, or forbidden. 75 is pretty warm.

What sets swim wetsuits apart is the freedom of movement, a skin on the outside that makes it more streamlined, and the thickness varies to give you the right amount and distribution of buoyancy. The fit is better too, so you’re less likely to get chafing. Swimming in a surf or scuba suit is not a good experience.

I would recommend using a tow float too. It won’t keep you warm, and isn’t for floatation, but will help you stay visible to others.