To avoid driving the car into the water use a bar extension, far less stressful and saves the car from salt water and also getting stuck on a slippery slipway. No extension bar use a rope.
That trailer doesn't seem to be set up that well. With my boat that is 6.8m long, I can push it off the trailer with one hand and launch and recover on that slip without getting the wheel bearings wet!
Where you launching into a lake? I am thinking of buying the 550 for a lake and was worried that you could only launch them into the sea as a lake ramp might be too shallow.