Yeah, the chilling in bogs thing would have been extremely gross to the Romans upon arrival. British bogs have a lot of peat and are much more sterile due to being acidic and having low amounts of oxygen in the water. Bodies can be preserved in bogs for centuries. Relatively safe for chilling in on a warm summer afternoon.

Swamps the Romans were used to were riddled with disease and parasites, taking a dip could give you a whole tasting platter of fungal, viral and bacterial infections, a dozen leeches and god knows what else