Doesn't have to be completely submerged, it just needs moisture and air and once it sets in, it doesn't need as much as it did to start. Teak plywood is usually only teak on the outer ply, so teaks rot resistance doesn't really protect you here. Lockers can be bad for rot/fungus and mold since they are not usually well ventilated - any moisture that does get in there stays in there. That little bit of extra moisture and humidity can be enough to keep the rot going, but it still does need some elevated moisture - in other words, if it were completely dry, it wont rot, but it doesn't need to be dripping wet either - actually in my experience, rot seems to work faster if its not constantly submerged.
Once you fix the bulkhead and the leak, make sure the locker is well ventilated. LIkely its been there for a while - maybe the outer teak plys were the last to go so you didn't notice until now.