There isn't much data on ditching a Gulfstream. The only case of a water landing was the case of an aircraft flown intentionally below glide path into a lake. See: GIII VP-BLN. In that case, with no cabin preparation, the aircraft filled with water so quickly the passengers and crew were not able to don any flotation gear.
Everything here is from the references shown below, with a few comments in an alternate color.
Preparation is the key here, once you turn the airplane into a boat-soon-to-become-submarine, you won't have a lot of time. For more about this: Ditching.
[G450 Airplane Flight Manual, §4-19-20]
No tests or actual ditching have been made. The following procedures will improve the chances of a successful ditching:
NOTE: If RAM air is inoperative, select BOTH ECS PACKS-OFF.
NOTE: Power levers should be closed, airplane rotated to normal landing attitude (airspeed approximately 1.25 VS), slightly nose up, and contact made with water with minimum forward speed and at minimum sink rate.
NOTE: At water entry, expect the airplane attitude to be approximately 7 to 8 degrees nose up. Airspeed at water entry should be just above stall speed. This is to minimize structural damage following water contact.
NOTE: The airplane may be expected to remain afloat if it does not sustain serious damage on landing.
There have been cases of low wing aircraft remaining afloat for days and others submerged faster than the occupants could evacuate. The amount of fuel in the wings, I would think, would be a major factor.
NOTE: For passenger accountability, life rafts go out same side of airplane.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013
Gulfstream G450 Operating Manual Supplement, G-450-OMS-02, Extended Operations (ETOPS) Guide, Revision 2, April 2, 2009
Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011
Gulfstream G450 Quick Reference Handbook, GAC-AC-G450-OPS-0003, Revision 34, 18 April 2013
Gulfstream G450 Weight and Balance Manual, Revision 3, March 2008
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