Im not sure what the appropriate board would be so I'll post it here.

Is it possible to let a unit travel on both land and water. I was thinking something like the Marine's new AAAV.

This is the right place. :)
I think it is possible, but I don't think you can get the AI to understand how to use it. Not 100% sure though...

All the better I don't want the AI to be able to use it.

You simply select the unit type as 'air', but don't give it the immoble flag as the standard civ3 aircraft do.

Then I can set it to carry foot units only? I want it to carry marines. Also can I give it its own Amphibious Attack? I just got the editor working and dont know how to operate it that well.