As this WiiWare download's title suggests, you play a diver in the Medes Islands, which are just located off the coast of Catalonia (though of course, you already knew that... just like we did after a quick visit to Wikipedia).
According to your diver's diary, there are loads of treasures hidden in the surrounding Mediterranean Sea so naturally you're going to have to swim down there and gather them all up.
Dive is split into ten different levels, each of which looks surprisingly different considering they all take place under the sea.
Some feature nice lighting effects as the sun splits the surface of the water, others see you swimming through dark and narrow caves with only your torchlight for visibility.
As you swim through each area looking for treasure, you need to keep an eye on your oxygen meter. This reduces both as you swim and as you make contact with any creatures under the sea (presumably they all inadvertently unscrew your oxygen tank as they bump into you or something). The aim, then, is to get to the main treasure on each level (marked with a yellow dot on your radar) without losing oxygen by taking too long or hitting too many enemies.
This becomes a much harder task in later levels, as the numbers of sea creatures greatly rises and you find yourself trying to skilfully navigate past jellyfish, squid, sharks and the like. If you've played the infamous underwater stage in Teenage Mutant Hero Turtles on the NES, this is a modern version of that.
Despite this, Dive only occasionally gets frustrating. The controls, while fine for the most part, aren't really accurate enough to allow for precise navigation through some trickier areas, but they're good enough to get you through most sticky situations without too much hassle. You're also armed with a harpoon gun which, while not exactly turning you into an underwater Rambo, does help you get out of the odd predicament.
If you're good enough you should be able to get through Dive's ten levels in four or five hours, but the need to go back when you buy better diving equipment and explore further, deeper parts of levels you've already cleared is a clever way to extend the life of what's an impressive, relaxing adventure.