that's wood rot, sorry. Serious removal of infected timber, replacement, and treatment of surrounding area. Then remove the cause - probably freshwater leak.
There are many chemicals that can be used to kill the fungus but it's plywood and should be easy to replace. Before replacement and after removal use one of the fungus killing chemicals around the area to make sure all fungus spores are removed.
Removal ASAP is essential to prevent it spreading, also the water leak needs finding and repairing.
+1 although it's nothing like so rapidly invasive or pernicious as dry rot. You need Sigourney Weaver equipped with high-tech weaponry for that.
Sorry about the bad news, ifoxwell, but it could be worse.
Replace with good quality exterior or marine ply, epoxy-painted (esp on end-grains). Easiest way of bonding each board with its neighbour may be also be epoxy, with glass cloth, depending how it's all constructed. In fact you can see evidence of the same technique in the second picture, but probably with polyester resin rather than epoxy. Any other areas of the boat showing excessive wetness will need investigating, too.
From the look of it, that locker floor could do with a drain, so that any water that gets in has somewhere harmless to go.