My 2 cents,
I find it hard to believe that both engines are frozen unless they both were under water. They are liquid cooled with a closed loop, so even if you don't winterize, the most common issue is a cracked exhaust manifold and that usually doesn't result in a seized engine.
I would pull both pumps before you make the determination that the engines are frozen. The pumps if not serviced regularly will seize as they are in the water. No grease or water intrusion in salt water and it is simple for them to freeze up.
Watch out with any mechanic if you know little about these boats as they could tell you anything and what would you know.
Take a bunch of photos and feel free to share them as many here can offer very good advice.