Proposal: Cultural Tourist
In the rule “Routes”, after the text “the Destination it had when it was In Transit during that Setting Sail action”, add the following text:
, and if there is a set of Tenets in the Culture of the Location of that Boat that is not in the Culture of the City that owns that Boat, a publicly randomly selected Tenet from that set is added to the Culture of the City that owns that Boat
Tenets in the Culture from other Cities gets copied to your Culture automatically and mandatorily through your Boats, as suggested by Josh
DoomedIdeas: he/him