Seems to me that if the owner knew of the problem and didn''t tell you, you could sue in small claims court to get your $700 back.
If you didn''t ask for prior surveys before haul out and your own survey, might be good to do next time around.
Another possibility to save on this expense is to ask the surveyor to do a survey while the boat is in the water. If he nixes it then, you''ve saved the haul out cost. If he OK''s it but for what he can see after a haul out, it may cost a little extra to get the surveyor out a second time, but it seems like it might be worth it. Could also do the sea trial before hauling out--don''t like the way it sails, no haul out, no surveyor.
Good luck--and in my experience, I think you ran into a rare bad apple. The boat owners I''ve encountered while shopping have been open and shared all their documentation, including prior surveys. And for a 1985 Hunter, I think you could do without the broker and the broker''s fee and check out the marinas and classifieds.