I would be surprised if the end of that bolt was actually protruding into a water jacket. I have heard of this being done, but it is not the usual way.
If it is broken off too far in there, it is possible that the drill-outs might not work, since the 'collet' part might hit the casing before it is able to engage the fastener. One shortcoming of this design...
If it weren't for broken and stuck hardware, auto repairs would be at least 50% easier.
1986 300E 5-Speed 240k mi.