Thanks for the response Mike, Randolph and Terrel.
Yes, the boat slip contibutes to value. I have a model match, closed sale matched pair within the past 12 months - 2 model match properties in the same project that closed on the exact same day. One with a boat slip sold for $275,000 and the one without sold for $260,000. These boat slips are in a great location for boating- they are in fact the very reason some (but not all) people buy in that condo project. The project has some other very desirable features also. I was suprised a higher value for the slip was not indicated by the matched pair. Of course, that is just one pair of sales...
It would seem you can support a value of $15,000 for the boat slip. I would use the garage line (and explain.)
I.E. 1 gar, 1 B.S.
That slip adds to the appeal and marketability. You should make sure that ownership of the slip is included in the transfer and to the security of the note and deed of trust. (or mortgage)