markusbeutel
NiGHTS
Yes, yes, yes, yes & yes.
Yes -- vanilla work boats can go onto ocean tiles once you've got the Astronomy tech.
Yes -- cache deleted before installing NIGHTS
Yes -- ModUserData folder contents deleted before installing NIGHTS
Yes -- file integrity verified through steam
Yes -- only one copy of NIGHTS installed
Alright - turns out there's a simple answer to this: You need the Compass tech, not the Astronomy tech. Compass is an either/or prerequisite for Physics - so you probably haven't researched it in your game, (as taking Mathematics instead of Compass to get to Physics is a shorter research cost path, which the game always prioritizes if you don't manually select each specific tech).
If you have researched Compass, then I'll have to do some more digging to find out what the exact problem is - but I'm betting this is it.