It
is imaginable. I have had loads of engine failures and a ditching (though, ironically, not as a result of an engine failure.)
There is no question that the passenger should remain in his seat, firmly strapped in, otherwise he might be killed in the impact, especially if you misjudge and hit the swell leading edge.
The question of popping the door is more difficult. We certainly had a heart-stopping moment as the aircraft sank when we thought that we were not going to be able to open the door, but we did get it open with the application of considerable force. That is an argument for popping it.
The argument against is that it will affect the flying characteristics of the aircraft, and the last moment just before you try to judge it perfectly onto the crest of the swell isn't when you want a sudden swing to the right, and increase in ROD or decrease in airspeed.
I would suggest that you make your mate "door monitor". It is his job to familiarise himself with the handles and be ready to open it the moment you touch down. This is particularly important in a high wing type, when the fuselage will be submerged immediately.
If you have an axe you should make sure it is to hand.
Or, if you are now sufficiently scared...go in a twin
Timothy