The original Endless Ocean saw you stepping in the shoes (well, the wetsuit and flippers to be precise) of a diver who was tasked with exploring the depths of the seas and identifying a wide variety of different fish and other creatures. It was a relaxing, enjoyable game but our only concern was that it took a long time for the title to get into proper game-based 'missions' and even then there was never any palpable sense of danger, even if you came across a shark. That was the point after all - it was a game about relaxation, not death and destruction.
Conversely, the sequel, due to arrive on your Wii early next year, focuses much more heavily on adventure and exploration. You'll be checking out sunken ships, trying to find treasure and not only encountering sharks and salt water crocodiles again, but actually having to evade attacks from them this time around as well.