Generally around the high 70's to low 80's if you're sitting in the sun, a little cooler if you're in the shade. There will (almost) always be a nice breeze when you're on deck.
During the day, you'll be fine in a swimsuit or t-shirt and shorts.
At night, you MIGHT need a long sleeved shirt or light sweater/hoodie if it's windy, but I doubt it.
If you plan on lounging on a deck chair in the evenings, there are always blankets and towels available if you find it chilly.