I've investigated the site, and I can verify that the mine is not in the river, but safely on the northern side of it.
This was quite an unexpected consequence of the prospecting skill, though, so I've taken the trouble to revive the character. Well, tried to, at least -- reviving dead characters isn't really meant to be, so there may be things that have gone wrong in the process. Please tell me whether it worked.
Swimming toggle will be coming. :)