The boat shoe is the well-dressed man's flip flop. It will easily take you to the beach, the pool, the boardwalk, plus a million places a flip flop shouldn't go. It pairs with shorts in a way few leather shoes can, and looks equally sharp with a pair of jeans, corduroys, or chinos.
Some guys have trouble with boat shoes. They think they can't pull them off, they're too preppy, or too "old man." The only reason for this is because we never really wear them as kids, but when you get to a certain age, flip flops and sandals just don't feel appropriate in a lot of the situations we find ourselves in as adults.
I'm not going to wear a pair of Havaianas to my boss's barbeque, and I'm definitely not going to sport shorts with a pair of sneakers and atheletic socks. Few things in style may seem simple, but when it comes to which shoe you should pair with your shorts and chinos in the warmer months, it couldn't be simpler.
The best part is they're super comfortable too. I always wear mine sockless if I'm going with shorts. It may feel unusual at first, but you get used to it pretty quickly. Be sure to keep a can of shoe doederant spray on hand to keep the funk down. I like to spray my feet before wearing them, and then the inside of the shoes when I take them off.
A majority of the boat shoes you'll encounter come from one of the two founding brands: Sperry and Sebago. Both have a full collection of styles and colors, and most run in the $75-$90 range. A pair from either of the brands won't let you down, but the design is pretty universal and you won't have any problems if you pick up a cheaper pair from a different brand. I've had a pair from Lands' End for a couple years now. I wear the hell out of them and they still look great.
Trying to figure out what to wear with your boat shoes and shorts? Check out Primer's Complete Visual Guide to Shorts.