The (tropical) mangrove tree is a very important part of coastal ecology-this tree lives in salt water, and it accomplishes many things:
-it protects the coasts from erosion
-its roots are used by shellfish for attachment
-it provides breeding grounds for many fish
-its dropped leaves feeds and fertilizes coral reefs
The tree is so important, that when it is removed or destroyed, local fishing drops off drastically.
Suppose we could engineer a mangrove that could withstand freezing temperatures? Then the tree could grow on all kinds of coasts-possibly all the way to Maine. Would this be a benefit? We might be able to recover the shrinking Atlantic fisheries-more spawning grounds, more shellfish, etc.
Or would we unleash an ecological disaster?
I’m voting for ecological disaster. Mangrove swamps are very fertile and productive because the organisms which breed / live/ thrive in mangrove swamps evolved to fit that environment.
Some cold-water organisms might adjust to the sudden introduction of engineered mangroves, but it would certainly disrupt the established ecosystems in the biomes, and we would have no way to tell whether the survivors would be useful to us or more “trash” species.
The northern Atlantic fisheries are frex the Grand Banks, and not at all coastal. Though stabilizing coastline with mangroves is an interesting idea.
When we went to Key West last spring, I got to actually see mangroves [we went to the area next to the blimp station on Cujoe Key to look at a feature we spotted in Google that we couldn’t figure out from the image and were going past the area anyhow 
] They are actually fascinating, it reminded me of a bit of shoreline at the cottage we had when I was growing up that was lined with willow trees [obviously fresh water not salt:p] that had roughly the same function of holding the turf at the shore together, and had tiny crabs and crayfish, and minnows hiding in the root ends waving underwater.
I wonder if we turned some of the ‘salt deserts’ created by over irrigating into swamps if mangroves could survive there and help filter out some of the excess salt.