Waterfront in Fl is incrementally valued based on location...oceanfront being most desirable and obviously carrying a higher price tag. Following is oceanview, then Intracoastal Front, Intracoastal View, MarshFront, Marshview and Lake Front, and Lake View...except in the historic district which is what you are referring to.
It depends on which side of Maria Sanchez Lake you are referring to. On the East side of the Lake, those homes typically on Cordova , Charlotte, or St George Street are in the historic district and have waterviews. These houses hold significant value (historical as well as $) and maintain it and will always have appreciation if kept well. Many of these homes have been lovingly restored. On the Western side of the Lake, along Washington Street, some of those homes have not been kept up well and some are in need of remodeling. Because they are more of a "direct" waterfront property in a historic area (bordering Lincolnville) they command more $ than those say on a lake in a subdivision. For instance 314 St George Street built in 1917 with 2359 sq ft sold in Jan of this year for $675K after being on the market for 212 days and last list price of $698K. The restoration of that home was spectacular and true to the architectural elements of the orgininal home. There is a house at 145 Washington Street that is in need of total gutting and remodeling and probably would be worth the $500K price tag and more if it were restored to its original quality given the sq ft. and the direct frontage on the lake. 153 Washington Street sold for $709,000 with 2295 sq ft in 2006 and 143 Washinton St sold for $858,000 in 2007 with 3055 sq ft. You can do the math based on sq ft and sold price to get price/sq ft. But roughly it is av $286 to $290/sq ft. If you take that number an multiply times the sq ft of the house you are looking at and couple that with the condition of the house and the repairs you'll need to do ....It very well would be worth it!
I hope this helps you. If you are working with a REALTOR that person can confirm what I have shared with you. If you are not working with a REALTOR, let me help you buy that house! I would.