Swimming shirt?

Most tri tops have panels of slick material... supposedly the water slides off them. It's a balancing act between having those and having enough wicking material for after the swim.

To not impede your swim too much, get something tight.  The loose material dragging in the water is where the real drag comes from.

No shirt or rashguard is going to insulate you from the water.  You'll need neoprene for that - i.e. wetsuit.

If you're getting burned, can't you just swim in the morning/evening with sunscreen?

65~75F I'm in my sleeveless wetsuit because I'm a cool water weenee!  If you can tolerate the colder waters maybe a 1 or 2mm rash guard surfers shirt will help you.  But I couldn't imagine swimming with anything that's a loose fit though.  My son has done swim meets with a tight fitting sleeveless Under Armour shirt.  I haven't been to a local surf shop in awhile, maybe they have something.

When the sun is up I'll swim with a trisuit for protection.

Also there's always something like a shorty wetsuit like this when the waters are cooler.  My buddy has one that I've used a few times.