You've obviously never built a quadrotor that's better than toy quality. The parts alone are around 800 if you buy them non-wholesale. Of course I would expect that price to come down if they seriously mass produce them.
Let's start at the weight 4 lbs. WTF are you guys thinking?
Again,. that's just what a quadrotor that can carry a go-pro weighs. The phantom's slightly lighter than that, but it's underpowered for the job and only flies for like 7min. (that's what I hear, I don't have a phantom).
Completely waterproof so the battery bay is watertight? The motor stators and windings are watertight? The flight control board is also watertight?
Funny thing, brushless motors are inherently waterproof. In the underwater ROV community it's common to use brushless motors as the propulsion. Honestly it's probably IP 66 or similar. It's like your car, protected from water ingress so it's good in rain, but not actually submersible. Why make it water tight? If it falls in the lake it's going to sink anyway.