When hunting our marsh I've fallen into the habit of vesting my Chesapeakes any time it's cool enough for them to be comfortable in it, both to afford some protection from hazards:
and because energy not expended maintaining core temperature is available for other uses:
That, and I like to keep my partner and pal as comfortable as possible.
As for the notion that a dog stays wetter under a vest, it just doesn't (Ready for this?) hold water. Our marsh blind is a ten minute ride from the boathouse, and by the time the vest comes off at the dock, the fur under it is virtually dry, while the uncovered neck and shoulders that were shaken out and exposed to open air remain wet.