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: Mishaps / G-III 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.

What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.


Analysis:

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: Abnormal Procedures & Techniques / Ditching.


Procedures:

[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:

    1. Distress Message . . . TRANSMIT

    2. Fuel Load . . . MINIMUM

    3. Cabin and Passengers . . . PREPARE

    4. Heading . . . PARALLEL SWELLS

    5. Emergency Power . . . ON

    6. Terrain Inhibit . . . ON

    7. Landing Gear . . . UP

    8. GPWS Voice Override . . . ON

    9. Flaps . . . DOWN (39°)

    10. RAM Air . . . RAM
    11. NOTE: If RAM air is inoperative, select BOTH ECS PACKS-OFF.

    12. Cabin Pressure Control . . . MANUAL

    13. Outflow Valve . . . CLOSE

    14. Differential Pressure . . . ZERO

    15. Stall Barrier . . . OFF

    16. Final Approach Airspeed . . . VREF

    17. Descent Rate . . . APPROXIMATELY 200 FT. PER MINUTE

    18. Touchdown Configuration . . . ESTABLISH
    19. 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.

    20. Fire Handles . . . PULL

    21. Life Rafts . . . REMOVE

    22. Public Address . . . EVACUATE

    23. Airplane . . . ABANDON
    24. 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.

    CREW DUTIES FOLLOWING DITCHING

    CAPTAIN

    1. Emergency Exit Lights . . . ON

    2. Remove life raft #1 from stowed position. Tie life raft mooring line to fastened seat belt at forward exit window, either left or right side.

    3. Remove forward exit window and deploy life raft #1 out window.

    4. Exit window and secure ditching line to fuselage for hand hold.

    5. Follow mooring line to life raft, board life raft.

    6. Direct passengers to follow mooring line to life raft. Assist passengers into life raft.

    FIRST OFFICER

    1. Emergency Exit Lights . . . ON

    2. ELT . . . ON

    3. Remove life raft #2 from stowed position. Tie life raft mooring line to fastened seat belt at aft exit window - same side as raft #1.

    4. Remove aft exit window and deploy life raft #2 out window.

    5. Exit window and secure ditching line to fuselage for hand hold.

    6. Follow mooring line to life raft #2, board life raft.

    7. Direct passengers to follow mooring line to life raft. Assist passengers into life raft.

    CABIN CREWMEMBER (if onboard)

    1. Assist Captain and First Officer in obtaining life rafts #1 and #2 from stowed position.

    2. Assist Captain and First Officer with securing life rafts by tying life raft mooring line to fastened seat belt

    3. Direct Captain and First Officer to open exit windows, secure ditching line to fuselage for a hand hold and deploy life rafts.
    4. NOTE: For passenger accountability, life rafts go out same side of airplane.

    5. Have Captain and First Officer follow life raft mooring lines to rafts and board.

    6. Continue to direct evacuation from inside airplane. Assist passengers.

    7. If time permits, gather first aid kit, blankets, food, garbage bags, etc., to take to life rafts.

    8. Depart via exit window and board life raft.

    References

    Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

    Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013

    Gulfstream G450 Quick Reference Handbook, GAC-AC-G450-OPS-0003, Revision 34, 18 April 2013