Slitherine

How are river boats supplied? Docking them in ports has not effect.

Normally, you just need to move whatever naval unit onto port hexes, and on the next turn they are fully supplied. Tell me the scenario/map you are playing.
One reason could be that the map designer has forgotten to change the terrain type from sea to port, or accidentally chose city type. During normal gameplay, another reason could be that at least three enemy units are adjacent that deny any supply/reinforcement.

Thanks for your reply Horst but neither one of those was the root of the problem in my case. The issue came up in a scenario I'm building. Turns out that it is not enough to specify "port" as the terrain type; you also have to place the appropriate flag on it or your ship will not be resupplied. Case in point is the Taranto Raid scenario: resupply will occur on the flagged port hex 17,19 but not on unflagged port hex 18,19.

Ah yes. Normally, the ports are flagged in scenarios. I would never dare to dock there with these forts around anyway.
Yep, it takes a while getting the hang of the editor, especially with rivers and railroad networks. Placing the graphic is often not enough.