CarlZog
Explorer
GlassJaw said:Last session was absolutely awesome.
Flattery, sir, will get you everywhere!

As you may tell from GlassJaw's comments, I think sea-based campaigns offer just as much variety and perhaps even more opportunities than land-based campaigns.
Since you didn’t indicate otherwise, I assume you’re looking at running a D&D sea-based campaign. If you’re running this in a home-brew world and you haven’t considered the sea much before, take a look at the seas in your world and the role they play. Land-based races go to sea to explore, gather food, transport goods, or steal goods others are transporting. Sea-based creatures treat the sea just like others treat land: a source of life, sustenance, territory, etc. Give these ideas some thought as you look at your maps and determine who in your world is involved in the seas.
How will your PCs fit into this? Do they all come from the same land? Why are they going to sea? Are some or all of them members of sea-based races?
I disagree with the advice to keep your campaign land-based or to avoid underwater adventuring. A ship-based campaign offers a LOT of opportunities. The ship is your base of operations and it moves with you! It can go anywhere, visit new places, etc. Heck, this is the basis of the original Star Trek series. Or people come to the ship, like Love Boat…OK, enough TV examples.

Give them and their ship a purpose, or let them choose it: Are they explorers? Traders? Pirates? Their choices will drive the nature of your adventures.
About DM tactics: Avoid the usual clichés of “the party is hired by …” A lot of their experiences should be more open-ended opportunities that they choose to pursue: As explorers, they’ve heard of tales of a magical archipelago worth finding. As pirates, they’ve acquired knowledge of a lucrative trade routes ripe for the plucking. As traders, they’re negotiating with a couple different Kings to create a new shipping trade between countries that they will run.
As for pirates, a lot of pirate games tend to default to D&D clichés: the party is hired or had a feud with another pirate. Pirates in RPGs seem to never be actually involved in the business of pirating. If they want to be pirates, they’ll have to go take some ships – a bloody, opportunistic business. Perhaps, they’d rather be pirate fighters, commissioned by the King to protect shipping interests by clearing the sea of the pirate scourge. Or privateers, authorized by the king to capture enemy shipping. Of course, there’s always a slippery slope toward becoming corrupt cops.

Underwater adventures depend on you not letting the underwater realm become just like air. Even with breathing spells, et al., there should still be an element of risk and danger associated with going underwater. Otherwise there’s little point to it.
There are a variety of d20 rules supplements for sea-based campaigns and adventures. Which one makes the most sense for you will depend partly on the kind of campaign you envision. Some are better for underwater stuff, some are better for piracy, some are better for swashbuckling type stuff, some are particularly good at ship-to-ship battles.
Like others said, give us a little more detail and we can give better input.
Carl