I like the theory that it is pretty noisy underwater, and agree that noise made in the water (wading, splashing, dropping something in a boat) are more problematic than talking, etc.
I like being quiet on the water for another reason - it improves the fishing experience. Its like how you want a crowd at a golf tournament - quiet. If I am quiet I can hear the birds, or the splash of a fish, etc and feel more part of the river.
I also have a pet peeve - powerbait users that slap the water with their hook to remove the powerbait. - I've seen several folks do this, including a fishing buddy (soon to be former fishing buddy if he keeps it up) and it drives me nuts.
Michael