Living in a landlocked state I was never much of a diver. But if you are an active SCUBA diver, you may find Dive Log (and companions Trimix and Nitrox) useful. Dive Log (iTunes link) does what you'd think: it logs your dives. If you've never been diving this may seem trivial. Dive logs, however, are far from trivial, as they contain crucial data related to dives -- like how much weight you added to your belt in a given location. Dive Log will import/export your data in UDCF format, and offers full sync with Diving Log 4.0 (unfortunately only on Windows). Plus, the developers have a free app for backing up your logs, and you can import logs from MacDive.
Nitrox Tools and Trimix Tools (both iTunes links) are more specialized diving apps for the serious SCUBA guy or gal, who may mix their own tanks. I'm certainly not one of them, but the tools look sufficiently hardy for those folks, plus there's an online version of Nitrox here (for Safari) that will give you a taste of the app's capabilities.