I visited the large NE one earlier making an educated guess based on your overview map. 4000 70 -4300 landed me in the water a few blocks from the beach.
I lit up the red mushrooms, but otherwise it'd be nice if at least that one remained a Mooshroom Natural Preserve with no structures and no mining (and no killing, of course). In fact, I'm calling it that.
No baddies spawn on the mushroom islands, so there's no need for a safety hut.