To easily get kills with the water vehicle, choose a recon class and get onto a jet ski (this works best as attacker on the rush map "Isla Inocentes". Drive to an area that gives you a good view of the objective and also provides cover (the little islands will work well). Then, switch to the passenger seat and turn your character around so that you are facing the objective. Enemies can now be sniped from the jet ski. Each kill you get with the sniper rifle counts as a water vehicle kill. This also works with the larger patrol boats, but using a jet ski is preferred because its small size makes it harder for you to be hit by an RPG and is much faster.