A little over kill on the size of the holes for the water pick up on that lower unit. Heck the ones on my V6 homemade lower unit low water pick up are half that size and only 2!
That looks like detonation to me. Number 4 looks like it was gettin starting to get eat up as well. Probably should be run in 93. The way the fuel is these days.
If you live, were the temperature swings cold, then warm through the day. Like in the North, then. It's best to fill the tank, so the walls don't condensate and fill your tank with water. If you keep your boat in a garage where there's no or little temperature swing, then you can leave the tank...