After the opening cutscene you're immediately immersed into action. Nathan and Sam are chased by several boats. Ram the smaller ones while heading straight for the island as much as possible. A larger boat will appear sooner or later; stay away from it and don't let it get in front of your boat. Dodge the explosive mine projectiles until one hits the boat anyway.
Swim towards the surface, then dive again as the boat comes towards you. Resurface when the coast is clear and climb your own boat. Take cover and start shooting your opponents on the other boats. When you've taken care of them, continue towards the island while avoiding the rocks.