If it was just one guy who thought "fuck it, it's hot, I don't have anything to do, I'm going for a dip in the bog" and as he got comfortable, some Roman legionaries walked past, looked at him, went back to camp and noted "The native barbarians bathe in swamps", it would be a good bit

But the bogs aren't necessarily bad places to bathe, all things considered. They're acidic enough to prevent decomposition of corpses, so they're probably more sterile than still water.