When you're in the sea, there's a large circular area you can explore around the boat, you only being able to go so far away from it (but you have plenty of space). There's no real goal to the game beyond exploration, there are hundreds of species of fish in the water (I think, quite literally hundreds) and you have to examine all of them three times in order to fill in their information in your log book, and there are also lots of exciting secret areas to find too. For example, you might just find yourself swimming through a vast underground cave... I shan't spoil any of the others though!
There is a very loose storyline about a woman who is searching for a mythical sea creature, but other than that, the gameplay is entirely open and you just choose a new place to swim about and explore every single time. The longer you explore for, and the more you do, all lead to you eventually getting improvements made to your equipment which allow you to venture into areas you otherwise could not access. You are also given the optional task of taking people down below the water to see their favourite sea creature. Plus, there's also the nice, though slightly pointless, additional feature of an aquarium, you own a giant tank which you can put a limited number of your favourite species in and then swim around with them.
On the whole, this is a very nice and open game. The graphics are very good, so the underwater wilderness all looks beautiful. There are no real constraints, you are allowed to explore where you want and when you want. The only down side is that I think it should be MORE open.
I'd rate it an easy 9/10