Couple of points...
The BV-107 photo taken on the North Slope provided a visual illiusion as the actual pitch angle was was less than it appeared.
The drag the landing gear would have at the 10-15 knots of water speed would be far less than at an airplane's takeoff speed of 70-90 knots.
On a Chinook....the allowable center of gravity travel is on the order of 144 inches depending upon the model of the aircraft....thus it has a full 12 feet of CG range. Add in the quad gear....spaced a long way apart and it does not want to tip forward like a tricycle geared aircraft.
In a boat hulled helicopter like the S-61.....I would imagine drag would be the issue and not control limitations. Perhaps Crab or some other 61 or Sea King pilot could enlighten us on that.
Perhaps control touch is more important than anything.....
YouTube - cubdriver749er.com - demo video 1
YouTube - Landing on water without floats!