On my very first sea kayak I made the gap between deck and bottom of rim 1/2". While it worked, it didn't work well; the spray skirt was a bit of a hassle to get "seated" on the lip. 3/4", as others suggest, or more is fine, very easy for skirt application. But on my latest boat I wanted to get the coaming as low as humanly possible for rolling, while still being very easy on the spray skirt issue. I used 5/8" clearance under a Vaclav/One Ocean style fiberglass/carbon fiber layup coaming. I much prefer the look of wood, but again, I wanted as low-profile as possible on this one, plus the wood rims were forever requiring R&R (repair & refinish) from the constant wear of routine neoprene spraydeck use.

The "ugly" coaming works so well I'll probably never use anything else from here on out.....

http://mysite.verizon.net/NGC704/newOI/Page_9.htm