Edit: This achievement has now become discontinued.
This needs to be done in one sitting in a Ranked match and as the description says "without dying".
I'd highly recommend just boosting this, find a partner to help you out and then get into the same game with each other. You only need one other person so once in the same game on a map with boats, meet each other at an agreed area or water and kill away.
If the opportunity comes up and you have a group of at least 3 or 4 people you may get a chance to Request a New Game. If you are quick enough and lucky to get a map with boats on then quickly take turns in killing each other with the boats weapon until your award pops.
Trying to do this legit will be tough! If you are in a boat by yourself you will have to change seat to use the weapon and the boat will just come to a stand still, which gives your potential target plenty of time to line you up and take you down.
A good map to use would be TheBlackGold but thats considering no one else is playing otherwise you will be hammered by choppers and Snipers.
Whichever way you decide to go for this achieve once your award pops, your achievement will pop after the game has finished.