No need to clamp the hose off inside the boat, there is a valve on the bell housing that pushes inward while the drive is installed to allow oil to flow. When you remove the drive from the bell housing, the valve is supposed to pop outwards just a bit to close/stop the oil flow. If it don't, then time for a new valve.
The drive oil system is gravity feed. When you drain the drive oil with the drive still on the boat, the bottle will/should drain too as long as the Merc oil hasn't sludged up at the bottom of the reservoir. (< oil sludge buildup is common.
Item number 32,
http://www.mercruiserparts.com/Show_...c=BELL+HOUSING
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