Sprig, I'm with you on using good judgement, re: hypothermia,
with any dog. A whooole lot of Labs and, yes, even some Chessies, lack meaningful undercoat. (How woolly is your boy's belly? And if he's fat enough for it to provide meaningful insulation...)
You would, however, apparently be surprised by what strong swimmers pointing dogs brought on to it can be. My theory being that lighter bone structure may help them ride higher in the water, and my observation has been that the dogs riding highest in the water have been the fastest swimmers. Last Brittany, Kie, was nearly twice as fast in the water as the first of my Chessies to share his time, albeit one of my noticeably slower two, and replaced him when we hunted marsh. Kie could hang with the third Chessie of his time, Peake (aka: "the coyote") which had to be one of, if not, the fastest of my five to date. Don't recall how Kie fared against the second of his three "brothers," which cancer took at 22-months, and never raced one of my previous pointing dogs against retrievers, per se, but the pertinent point is that they were also fine duck dogs, aside from the same weather concerns I'd have for my retriever's, per se.
Hoped I had a photo of that last Brittany Kie swimming with one of his "brothers," but the best I could do was him assisting the second, and ill fated, Blue's, maiden voyage:
Hard to tell there's a few feet of water in that duckweed choked boat slip, but this open water shot shows more of his buoyancy advantage over heavier boned dogs:
Little fart could scoot on land and water, and it tickles me when one of my hunters from that time period brags on dog work that turns out to have been my "spotted Lab's".