More than likely the head gasket is blown between the #6 and # 8 cylinders. The engine does not need to come out.
I highly doubt you warped a head unless the engine was overheated. The problem is that the head and/or block could have been damaged if the blown gasket is a result of detonation. Its not likely the case though.
If you are going to keep the boat for a while, I would have both heads pulled and have the heads redone with new seals, springs and valve job.
As far as the new owners quoting $1900 plus parts????? I don't know how they can do that until its apart and they know exactly whats involved.
A valve job, springs and seals will probably run about $500. All the rest of the gaskets and misc parts about $200. Labor is probably 8 or so hours.