You can just use a combination of 2 trigger conditions.
Say the point you want to check is called "TempPoint" then these conditions will work for deep water:
-
And - All Conditions are True
-

(Terrain pathing at TempPoint of type Amphibious Pathing is off) Equal to False
-

(Terrain pathing at TempPoint of type Walkability Pathing is off) Equal to True
(These are Boolean Comparisons)
This means the point is pathable to amphibious units but not pathable to ground units. Only thing that fills those two conditions is (you guessed it!) deep water.