Rick, thanks. I get the idea of welding through the hole, but there's hardly room there for the nut itself. It could work though. Here's the other issue - I'm almost certain this is not a stainless bolt. I don't think any of the bolts on the pipe are. This particular bolt might have been put in there by me (this boat has been in my family for a very long time). But I'm pretty sure the rest of them were factory (hard to be sure, been a long time since that pipe was installed). So anyway, the corrosion in there is probably really bad. I have worries about the easy out too. Mainly that even if I get it to go in there, I'll just break it off due to stuckness of the bolt. I have an arc welder but no mig. I think that will work on the plain steel bolt. I'm a little hesitant to weld in there given fire risk, but if I'm careful, I think it's doable.
Use brazing torch to minimize and concentrate heat on rounded bolt. You will have to burn out corrosion. I assume saltwater.... I also assume motor is out??? Heat up and spray with pentitrating oil. Be sure to have fire ext. near by. Let cool and repeat. Pray it is SS and not ferrous. If salt water boat, I suspect Y pipe is corroded badly too. I am having issues as well! Good luck!