First off, go get
TagTool. Its a little script for Gmax that gives you buttons to push to rename any selected part. It includes all the FS2002 tags, as well as all the ACOF tags that are recognized by MakeMDL.
Secondly, remember that you have to include the water rudder in the aircraft.cfg as a contact point.
This is from the Caravan Amphibian:
point.13= 5, -30.90, -5.250, -7.300, 1600, 0, 0.00, 50.0, 0.0, 0.0, 1.00, 1.0, 1.2, 0, 0.0, 0.0
point.14= 5, -30.90, 5.250, -7.300, 1600, 0, 0.00, 50.0, 0.0, 0.0, 1.00, 1.1, 1.5, 0, 0.0, 0.0
The first number is the class of contact point. FS will recognize "5" as a water rudder. As you can see, the Amphib has two, each capable of steering 50 degrees. The right one (point.14) has an extension/retraction time of 1.1/1.5 seconds. All this must be entered into the config for the water rudder to both show up and function correctly.