The marina is correct that if left unrepaired you could have a catastrophic failure. It has happened to others and I know of one who luckily was able to limp back to their marina with a totally broken gimbal assembly/ring. I would not use the boat until you have it looked at by another marine mechanic for a second opinion.
What make/model/year/hours do you have? That could determine if the boat is worth the repair investment.
So I also suggest that you check around with other marina's like Dave's Motor Boat Shop in Gilford
http://www.davesmotorboatshop.com/ or Andrews Marine
www.andrewsmarineservice.com/ There are others and someone may have another suggestion.