The best way to get this one is whilst you're going for your garden ops trophies when trying to complete it on normal, hard and crazy. ONLY use the maps shark bite shores and port scallywag, as both of these maps are pirate zombies.
You'll find it may well take you several attempts to get the three trophies mentioned above so try and go for crazy first which will pop all three, and Heave ho should pop if you only use these. Takes a while but eventually it'll pop.