I'm afraid I don't have any sage advice on the subject of ditching an airplane. The only Gulfstream to end up in the drink flew right in there on approach with no warning, landing next to a boat, and the occupants were all rescued moments later without so much as a life preserver. More about that: GIII VP-BLN. There seems to be a case like that every year or so, such as the case of Lion Air 904.
From the GIII incident we learned that getting the outflow (or TROV) closed prior to turning your airplane into a boat may save you from becoming a submarine. From many other incidents we see that air in the fuel tanks can help the airplane to float. Even if your tanks are full of fuel, which is less dense than water, you should be able to float for a while so long as the airplane remains intact without any holes allowing water inside.
What follows are just notes from various classes, including two flight attendant courses I was sent to evaluate. In the Gulfstream world the Quick Reference Handbook is a pretty good resource, so if you have time to prepare, start there. You should, of course, think about this as part of your regular study patterns
Everything here is from the references shown below, with a few comments in an alternate color.
Photo: Lion Air 904, from Lion Air 904.
Remember also that your best shot a communicating your distress may be on:
Pilots should don their life vests in advance
Flying a jet with a small cockpit window, high approach speed, and what might be a fairly high sink rate are you really going to be able to evaluate the primary and secondary swells? The Aeronautical Information manual goes on to caution: AVOID THE FACE OF THE SWELL. That is fine advice too, but if you are timing your approach to land on the back side of a swell, the cost of failure is quite high. Looking at the following diagram makes it clear to me that landing parallel to the swell is the way to go.
Figure: Wind-swell ditching heading, from AIM, figure 6-3-5.
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