Agreed about painter being secured before reversing. I always have a back-up rope to the bow U-bolt just in case.
Once you reach the water's edge, switch off engine and leave the car in gear. There's a fair weight with trailer, boat and engine. Most handbrake cables will stretch slightly over time, so this is good back-up measure.
With the 4.8, there's no need to dunk your rear wheels. She'll float off easily as long as waterline is up to mudguards on trailer. On retrieving, pull the transom bung and ensure the painter, winch strap and a back up chain/rope are secured.