The MIB is a Medium Inflatable Boat, flat bottomedwith no hard bits. The RIB is rigid bottomed as you describe and either 21 or 28 ft long.
The RIB, due to hull shape, could not get into the Chinook fully and was not claered to do so in the UK. Of course they may have driven up to the ramp and transferred pax, but don't see the benefit of that.
Gainsy - I have never seen a RIB in the back of one, perhaps this precedes my experiences. The Boscombe pictures I have access to do not show a RIB, its a MIB.
PPF - The Chinook will float quite happily at collective gound detent position which is positive pitch giving about 20 torque. By the way, this exactly the collective position that is used during ground operations following landing.
The procedure itself is quite straight forward and the speed is controlled by collective and water depth by cyclic - its best to give the crewman a good six inches in my experience
. The Chinook has a power down ramp facility, i.e, one that doesn't freefall under gravity, so you can for the buoyant ramp into the water.
HPT