Unlike most traditional sports, water polo takes place in an uncertain element that most people aren’t comfortable in (water). Teaching a basketball athlete to side shuffle is easy since they know how to walk, but teaching an athlete to scull in water polo is odd since it’s an unnatural movement for a beginner.
The more you can explain the “why” and “how” to an athlete on land, the easier it will be for them to pick up the skill when you are teaching in the water. For instance, when teaching the throwing motion, use a baseball pitcher as an example (by showing how you can do a similar movement in water polo). Teach them first on land and help them understand: no elbow down, ball behind your head and shoulders not open.