First of all I do not know how old you are ,What your driving record is like,what kind of credit rating you have,if you have any coast guard certificates or safe boating classes you have taken or your experience with boats at all in general....I am not being nosey --- simply making a point that any one of those things can eliminate you from getting insured.
It may simply be that the boat is too old and they do not want to take a chance on it or you. No slam intended!.. You might contact a company that insures old boats or antiques ,Again no slam intended..
Above all do not Lie to anyone about the boat--- It will come back to haunt you in the end.The advise your getting is all bad and do not listen to that crap!That is just my take on things and I may be wrong.
Keep looking and maybe you will have some luck.