The terrain type of a tile is a property that determines whether or not it can be traversed by an entity. For example, your water-related tiles (e.g. rivers and oceans) could be assigned a terrain type of “water” to both restrict watercraft to those tiles and prohibit characters and land vehicles from entering them.

(Tile's terrain types in the Voxel Editor)

(Terrain type for characters in the Voxel Editor) - All - Prohibited from… - Restricted to…

(Terrain types for objects configured as vehicles in the Map Editor - see vehicles for more info) - All - Prohibited from… - Restricted to…