To add to Chuck's very informative posts, for the benefit of the wheel plane pilots reading, the concept of power off or power on water landings, be they emergency or routine, is a little different, in that generally "runway" length is not a concern. Yes, there can be tight bays an so forth, but generally you're operating in larger areas, landing on the numbers is not required. Where the issue is overwater flight and ditching, it can be assumed that runway length is not an issue at all. Thus, glidepath control, and precise touchdown along the landing path is less important, making it more possible to concentrate on the lateral precision. Landing along the crest of a swell is somewhat challenging, but doable in that you can approach the landing area at an accute angle, set up on the crest of the swell, and then alter your course slightly to land along the crest. Cross wind is much less a factor. If you're ditching, it is the only safe way. If you're landing a flying boat, it will usualy handle the crosswind fairly well. If you're landing a small floatplane in water with swells large enough to be a factor in a safe landing, you should rethink your plan to land. Small float planes do very poorly in even as much as a 10" wave or swell height. I have been a passenger in a Cessna 185, where the pilot learned this very expensive lesson.
As said, in glassy water conditions, you will not be able to judge your height at all. If you have properly executed your approach, you will come to know your height above the water when it is zero and not before. Stalling onto glassy water cannot be planned so as to be at all safe.
Swimming in an inverted submerged aircraft is very disorienting. I did this many times while recovering flipped over float planes. The cockpit, which was so familar, is now anything other than familar. You know all of the features, but cannot find them (and it's dark, and you're scared). A fellow flipped his Cessna 170 over in front of me (on wheels) while I was waiting to cross the runway. I landed the helicopter next to him to offer assistance. They were not hurt, but the assistance he needed was turning the fuel off! The fuel selector was now on the ceiling, but darned if he could find it! And, that was in prefect conditions, with time to look for it!
Ditching training is well worth the expense. There is a travelling training course available in Canada.
If you're flying over water without at least a suitable life jacket for everyone aboard, you're both illegal, and foolish. If the water is even close to cold, an immersion suit is a must. There are many types which are fine for flying. Be it an immersion suit, or life jacket, pockets are good. Preload them with everything you'll need while floating. Once you're out the door, and the plane is no longer accessable to you, all the rest of your emergency equiment is also inaccessible. What you will have, is what you got out with.
There are certain minimum costs we must incurr to fly, safety equipment appropriate to the location of the flying is certainly one of them. You don't have to fly out over the water, so if you're going to, get the right gear!
Pilot DAR