My take -
Broadsides: Most realistic rules for ships. If anyone in your group is a fan of sailing and will get into the realism aspect, it might be good to use these rules or they might be frustrated at the lack of realism in the other books. The other books don't even take wind into account during combat, for example. It also classifies ships by class, making it easy to define what kind of ship technology is in your world by allowing certain ship classes - a very nice touch.
Seafarer's Handbook: Best inspirational and high fantasy oriented adventure material. Best coverage on underwater adventuring - but you already said that you don't care about that.
Seas of Blood: Most comprehensive ruleset, as it includes mass combat for ship boarding, merchant commodity prices, and some other nice touches that the other books lack.
Swashbuckling Adventures: Best source if you want to go full into a swashbuckling golden age of sail campaign with gunpowder, etc. as it is, essentially, a full d20 game with new classes to replace the D&D ones.
As regards rules complexity, if you are using ships to get from place A to B and have a few encounters mid-route, i.e. as a quick plot device, then the latter three books will enable you to run things quickly and smoothly, with Seafarer's perhaps being the extreme of this style. If you want the actual sailing of the ship to be part of the game, including tactical ship-to-ship combat that attempts to capture the unique feel of ship maneuvering, as opposed to making them move like spaceships in a vacuum, then the rules in Broadsides are better.
Any of the books will get the job done just fine though. It's a matter of what your expectations are. I'm a fan of low magic and a healthy dose of realism, so Broadsides is my preference.