That's a really short ride. You will get sweaty on hot summer days (>75F), but a change of clothes and some deodorant will handle that.

A backpack is going to make your back sweaty no matter what. I'd lean towards a rear rack and panniers instead.

Don't worry about energy levels. Biking is the most efficient human-powered transport, and I'm sure your legs are strong enough from surfing to absorb a short ride.

If you're riding to the beach, you may want to look at hybrids or even mountain bikes. Sand and road bike tires don't mix well.