I suspect your lawyer will tell you, at this point, say nothing about anything to them, and give them nothing including the time of day.
"we need to get our own surveyor in here" Too bad, they had that chance long ago. Keep you boat locked, and if the lock mysteriously is broken, add criminal trespass and forced entry to the list.
"can you put your "statement" in writing." Let your lawyer answer that, when and as appropriate.
For now, you're still entitled to retrieve your boat, and it sounds like they're entitled to nothing--except perhaps payment on your account. Don't let them goad you into anything.