I agree, it's probably a regional thing. I would like to think the OP's comps will help determine what/if any adjustment should be made.I see a lot of waterfront in my area and, no matter what the realtors say, most of the time the value is negligible. The guy that has lake frontage of 150 feet doesn't really have any more use of the lake than the guy with 100 feet and the market here doesn't really make a lot of differentiation in them. So the ff$ could be minimal or nonexistent. Just something to consider.
I think Howard's questions are valid, especially since the question definitely varies from region to region; body of water to body of wateralong what type of body of water is the frontage?, does frontage allow access? Dockage? Is the body of water navigable? Who controls/regulates the body of water?