Is it possible to modify the boat so that it's a car? Or amphibious so that the tank is more realistic?
Yes, but you need to understand how the vehicles in RM work to be able to do that.
The passability of boat and ship is determined by the position of the autotile in the A1-sheet. The details are in the help-file, and it's a bit complex especially for the boat (the ship is easier but also more bound to the tiles).
Because of that, you have two entirely different ways of making a land vehicle:
1) exchange of all sea tiles with land tiles in a special tileset
2) use of scripts to change how the vehicles are handled (there are a few exsting ones)
The first way works completely without scripts, but you need to make a special tileset - and that tileset will still limit the movement of the ship and boat, it will only make it look like the player is moving on land (and he couldn't walk on that "land" without a few other tricks as well).
With scripts, it depends on which script you choose to use for it - there are three or four if I remember correctly, and they handle the vehicles in different ways. So no one can tell you exactly how to use a script until you select which script you want to use (depending on its features and its compatibility to your other scripts)