All of these books will do the job very well, but they each have a different focus/strength.
Broadsides: Realism -- especially if anyone in your group knows about sailing (it's the only book that accounts for wind in combat, for example -- other books have movement mechanics more suited to spacecraft than sea ships)
Seafarer's Handbook: Adventuring -- It's got more content for game ideas, like port towns, sea hazards, and underwater adventuring.
Seas of Blood: Completeness -- It's got a few things the other two lack, like mass combat rules for ship to ship combat and trade commodity values.
Swashbuckling Adventures: A complete d20 game with support for renaissance era play -- It can be a bit of a mixed bag. Some bits may not mesh well with standard D&D but many pieces are really excellent for inclusion in your game. It has rules for all kinds of interesting fencing weapons and gadgets, firearms if you want to use them, support for encouraging people to be lightly armored if that's what you want, support for lowering the magic level in your game if that's what you want, and a ton (I think 70+) of interesting fighting school prestige classes. I think it has the most potential, but also requires the most DM supervision to work. However, this book is really for running a campaign centered on these themes, not inserting these themes into a more standard D&D campaign. There are lots of editing errors. The included rules for ships and mass combat are passable but not as extensive as books that focus on these topics.