Although I am currently an officer in the Navy, I have always liked the idea of a naval D&D campaign, where the players own a ship and journey the high seas looking for adventure, or better yet, having adventure looking for them. In one campaign that I ran as a young man, there was a fair bit of planning to take it in a more naval direction, but the playing ran out before the oceans came calling.
Delta hiding out over in his hotspot posted about a naval campaign, or more accurately naval campaigning and a con game he ran. I really enjoyed reading his post. One of the points that he raised was his discovery of naval combat rules in OD&D (which I don't think I knew even existed) and the fact that he feels those rules are better than just about everything that has followed--for D&D. Seems like a wide-open invitation for someone to put out material based on the little brown books. Of course, Raggi, is including naval rules in his Wierd Fantasy Roleplaying--I wonder if he is making use of this material?
I think that maritime adventuring presents a nice branch in the possible "endgame" of an early edition D&D campaign. If one assumes that the endgame was the establishment of a keep of some kind (for the fighter), temple (for the cleric), and some type of magical school or tower (for the magic user), a maritime campaign could easily "end" with the characters becoming a powerful force on the high seas. Instead of the fixed assets just listed, perhaps they build, or somehow acquire, a large vessel--akin to the capital ships of today, such as aircraft carriers, battleships, or strategic missile submarines. Maybe they could establish an entire armada of vessels. Perhaps the end is merely their ownership of an island where each can build the keep, temple, or tower.
In the pursuit of any one of those, the campaign would follow the same general path as campaigns that we are more familiar with. They start off as hired hands on a merchant vessel or on a small ship loaned to them by a patron to accomplish a specific task. As time goes on, they eventually obtain their own (small, relatively weak) vessel where they can take some ownership in following their own desires. Over time, as their fame (or infamy) grows, they start to take on more powerful foes, perhaps venturing beneath the waves in search of adventure. Acquiring larger and more powerful vessels (with greater numbers of sailors in addition to the normal henchmen), they become movers and shakers in their small part of the seas.
Another method of showing progression in such a campaign is slowly expanding the geographic scope of the characters' exploits. Drawing from the real world, perhaps the campaign is limited to the Aegean Sea in its early stages, eventually expands into the wider Mediterranean, and then, at the highest levels, the adventurers travel 'beyond the end of the world' and venture out into the Atlantic Ocean, where the environment is more dangerous (deeper water, greater extremes of weather, more danerous foes) and the area to roam becomes much greater. Adventuring in the Atlantic Ocean could even be further segregated, with the characters first staying close to land (exploring the coasts of Spain, Portugal, or Africa) and only later sailing straight into the open ocean to explore the Azores or Madeira.
Finally, I think it goes without saying, but I'll say it anyway: Homer's Odyssey is a fantastic template for a maritime campaign.