Also, submarines.
If not as vehicles, but as plot elements. A sunken sub could be a (main) quest location, and to reach it, you’d need a boat, proper diving gear and some swimming skill at the very least (if not liberal application of some aquatic mutagens to obtain superhuman diving abilities). The randomness of the sub’s location could also force building your own submersible vehicle if subs ever become a thing, so you could never be sure if the sub would spawn in deep ocean or closer to the shore in shallow water.
Might we have underwater caverns? Underwater caverns with airpockets?
Certain real-life deep sea aquatic creatures are terrifying enough as they are. I can’t imagine what their zombified version would be.
Since this is New England we’re talking about, hitting an ocean would happen in the east, right? And that might have to be the end of the map generation right there. All land should be to the west of the ocean. But, that’s just for the ocean, not large lakes, which might be more suitable.
If we take the ocean idea further, it could be endless from the east coast to the east, or it could just go on for ludicrously long time and eventually the player would hit Europe and find a vastly different ecosystem. Reaching Europe could be a thing - grab a boat/sub, fill it with tons of fuel and food, sail to Europe. Reverse Columbus. 
