On Lost Island if you go into the water off the beaches only a few yards out. Right along where the ocean drops off to the deep there are flat coral reefs that you can harvest for fungal wood. These reefs aren't to be mistaken for the spiral ones, those give you rock. They have to be the flat ones. My oxygen lvl is only 400 and It doesn't drop much and the water isn't that deep. It shouldn't be too hard for new players to collect and it doesn't require a diving suit. I can't say these reefs are located all around the island as I haven't looked but for sure the west side especially not far from the game made rock tower that sits up on the mountain. I can't imagine they wouldn't be all around it though.
Personally, my favorite place to get this stuff is on Genesis. In the plains area of the bog, the trees with the weird orange tops give lots of fungal wood. It’s safer than Ab or Val or even CI because you can easily see any dangers coming at you. The worst things I’ve seen there are Kapros and barys.