No water in the oil prior to the rapid disassembly. In the end, it was running fine before I tore it down, but it was getting tired. The oil pressure was getting a little "soft"- 40-45 psi at hot idle vs 50, a little more blue smoke on hot starts, that sort of thing. This is a .040 over motor, 1973 block. I know cracked cylinder walls are a bit odd for 360's, but the shape of the deformation of the piston top is what's leading me to think cracked wall being the cause. There's definitely a crack now.