Would it be possible to set this as an option? My players have been littering the server with cars in the ocean and rivers. Boats piling up at the oilrig. Minicopters all over the map.

It is possible to implement a hybrid of TC range and permissions. Currently, you can choose only one or the other. Originally this plugin didn't any have options for permissions due to the reason you stated, but people asked for permissions exclusively even though it probably wasn't good for them, and so I implemented it like a dummy anyway.

Oh. I must have misunderstood a setting then. So do I set "DecayMultiplierNearTC": 0.0, and revoke permissions?

Yes, you can choose simply the TC multiplier. That was originally the way I intended the plugin to work since it has low chances of vehicles decaying at the bottom of the ocean and such.

Okay. Got it. Thank you.

In order to improve the documentation, can you tell me which part was confusing? Did you think all of the types of decay protection were enabled simultaneously or something?

This one was all on me. Not gonna lie. But if anything I guess I would suggest saying something like 

If you want to restrict decay protection to the tc radius do not use permission and reffer to the configuration section of the documentation.
Maybe put that under

"Granting the following permissions to a player will cause their owned vehicles to not decay under any circumstances. You can grant permissions by vehicle type, or for all vehicles with a single permission."