About a monthnago I researched the premiums of resales across the metroplex during the last 9 quarters. The summary showed for lots that back up or front onto a lake, the average premium is roughly 22-28% on a per s.f basis. Additionally, lots that back up or front onto open space have averaged a premium of 6-10%. The data showed that these lots also sold faster, on average about 5-10% faster than a "typical" lot. These are averages across a few cities so before you put a figure on a specific property you need to look at the adjacencies because these figures will vary based on many variables, consult a REALTOR to get a specific market summary for the area you are interested in.