It took a few tries, but I did it. There aren't any soft kill boundaries in the water, but once you go down a certain depth, it kills you.
If you are looking for a true under water map to forge on, I have broken maps on Awash where you spawn under the water and can forge with the islands floating above.
I also have a water map on skyward that has moving water and land masses. This makes interesting infection maps with fast moving ghost like images.