I appear to have fixed it!
The problem was in this rule:
Before doing something other than examining the figurehead:
if the location is in the currently sinking deck:
now the going nowhere boolean is true;
say "You can no longer see the figurehead through the murky water." instead;
[...]
Of course, “doing something other than examining the figurehead” applies to every single other action, including the implied >LOOK that happens when the game begins. What you want to say here, to prevent the player from doing anything with the figurehead, is “before doing something other than examining with the figurehead:”
Or, that’s what you should do in the syntax of Inform 7 build 6M62. Apparently 6G60 understood things differently.
Thank you all for your assistance in diagnosing and treating Captain Verdeterre’s Plunder, which I am pretty sure is completely fixed and perfect now.