so you got water underneath it, and plan on putting a helicoil in it?
i wonder if all that will seal water tight?
if it were me, i would drill it out first and pound some sort of plug in it
(i remember ages ago i used to work at a place rebuilding small VW diesels. there was one particular block that was prone to cracking so we'd pound an alluminum plug in it and reroute the oil)