To facilitate your travels on what must be quite an expensive holiday, you have to take various people on dives and show them particular types of fish. These missions pop up on the boat's email system and tend to move the story forward, awarding you new abilities or granting access to areas that were previously inaccessible.
The more you discover, the better the equipment you'll earn and the closer you'll get to 'completing' the game. There's also a big book of undersea species that you need to tick off by identifying all of the different types of fish, leading you to the ultimate goal of finding one particular, unique creature.
Befriending dolphins gives you a free companion to take on dives, and they're quite handy for locating the buried treasure that forms one of the sub-quests. Alternatively, you can link up with a real friend over the net and share the experience of a dive.
While it's not exactly the most compelling two-player game as it is, you can always give your companion something to do by marking rocks with coloured paints or drawing messages in the sea. We think it's a lot better suited to solo exploration, but the option for two-player fun is there for those who want it.
Since you can only swim a limited distance from the boat on each dive, you have to keep surfacing in order to explore areas at the edge of your range. While this isn't a problem in itself, and you get to see some excellent animals that hitch a ride during your boat journey, the loading time when you return to the water is nothing short of epic. Patience is a virtue you'll most certainly need.