The thing that the water pump bolts onto

It might be just leaking through the bolt threads. I don’t think this is your problem, but I have seen serious cavitation pits in my cover. Now I’m spinning it much higher and I also have a small AC water pump pulley which is also spinning the pump quicker. The pits had increase at a rate to be a concern. I installed an anticavitation plate on the impeller and cut off 4 of the impeller arms and haven’t had it apart to see if it stopped.