Before giving up for today, I took apart the sauna and shifted a bunch of rocks to expose the crack in the hot water pipe. It's going to be a pain in the ass to weld there, the crack is in a tight corner and of course on the bottom.
Access is limited, there's a wall on two sides and I'll have to bend myself in there. Maybe I can remove the wire mesh for better access, but probably I'll have to take out all the rocks then. Argh.