Added value from canal

[Eric Beck]

Looking at a property with 125ft of canal access on the backside with enough dock space for a 30ft boat.  Located in zip code 32257 and the canal has direct access to the St. Johns River in Jacksonville, FL.  Think the property is overpriced, but how much value do you think the canal access would add to the property?  It's a bit unique in not many homes in the area of this feature and even fewer of them have sold in the past 2-3 years that I'm aware of.  

Speaking in more general terms, when you have properties with unique characteristics, or not many comparable that are relative, what steps do you take to value the property?  Seems with properties like this it's 50% science 50% art and just know the area very well by seeing lots of property.