This achievement may sound a bit daunting at first, but in fact it is pretty simple.
While being in the Rocket Boat, and as soon as you have a few lock-ons (preferably the smaller enemy boats as ships take a couple of hits) , just tap the
button to "jump" out of the water and immediately spam the
button.
You will eventually hit and destroy a few.
Enemy choppers do count as well.
I haven't found a way to keep track of the enemies killed.
Just keep repeating these steps (B, then LB) until the achievement pops.
(Please note : the clips were recorded by Xbox One's Game DVR. Therefor, any eventual unlocking achievements appeared on my screen were not registered by the Game DVR)EDIT: as pointed out by Tuschi, you need a boat delivered through the Rebel Drop List.
Personally, I'd like to add that you must have the MOD active.
In order to do so, you might to earn a few gears on some sea vehicle challenges.
Any suggestions to improve this guide are welcome!
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Good luck!