Got a DJI Phantom last December. Great for taking aerial footage. Stress levels do go up flying over water though.
The basic Phantom has a bracket for attaching the Gopro in the waterproof housing. However the footage will most likely be poor due to the quadcopter moving about (pitching and rolling) . The answer to that problem is to add a stabilized gimbal system to hold the camera.
I tried without a gimbal at first and I'm glad I ended up getting one. Makes a huge difference. However, the Gopro cannot be in it's waterproof housing. Weight is everything. The motors have to move the camera to stabilize it. More weight, the bigger the motors. All ends up costing flight time.
Not sure I'd want to be tying anything to the quad. Think you'd end up plucking it out of the sky or tangling it in a rotor.
Most of the quads of this size contain a flight controller of some sort. They use GPS and accelerometers to stabilize the quad during flight.
If I had to get another one I'd consider the DJI Phantom 2 with Zenmuse gimbal. I've also seen people coat the whole thing in some kind of "waterproofing" coating and crash them into the surf. While the did seem to survive I've no idea how long for...