If you choose a latex finish, be sure that it is not something that is prone to cracking and peeling. My personal preference is for oil based finishes outdoors. They generally look more natural, hold up better to the elements, and are easier to maintain over time.
No matter what type of finish you are looking at the two key things to look for are pigments and binders. Pigments protect the wood and binders or resin holds the pigments in place. The more harsh the environment the more protection you need if you want the finish to last.
This ipe boat dock was finished with Woodrich-Brand Wiping Stain. This product is loaded with both pigment and resin so it is easy to maintain. You just have to keep it clean and wipe it down with fresh stain when it starts looking a little dried out from the sun. The nice thing about the high quality transoxide pigments is they allow the grain of the wood to show through so it doesn't have that "painted" look.