Experiment With New Firing Mechanics
In Black Ops 3, your gun is always out. This means that you're now able to fire your gun while you're sprinting and while you're underwater. This is new, even for veteran Call of Duty players (like me) so it will be important to experiment with these mechanics during the beta.
Try using the water to your advantage and start getting used to opponents being able to zip around the map shooting at will. The faster you get used to the new firing options, the better off you'll be when the final version of the game arrives.