Shallow water drug

Can someone give me a quick quantification of how much shallow water affects speed in the flat current-less water context?
I have read the theory, but interested in rough reference points, such as "I lose X km/h at depth less than Y feet" ?

Also I read something about "popping" the boat which I understand is sprinting up to a certain speed where the drag effect changes somehow. I might have noticed that on a small lake I practice at, after a short burst it seems easier to maintain higher speed for half a k or so. It drops off eventually, but if I don't sprint, I seem to be stuck in a rut where the boat just doesn't go and the effort is not much easier than going faster after a burst.. But then, I might be confusing things here.

Thanks.

Good reading here - it references canoe racing, as shallow water is much more common in canoe races, but the same physics apply to skis. 
forums.paddling.com/t/canoe-racing-poppe...he-shallows/52573/18
myccr.com/phpbbforum/viewtopic.php?f=49&t=32683




Here is the most factually useful post from the thread in the 2nd link from John Winters, a respected expert in the field

Wave speed in deep water is a function of wavelength but, in shallow water, depth governs wave speed and, consequently, wave length. When a fast-moving hull encounters shallow water, the waves created by the hull shorten and grow steeper. This causes an increase in resistance well known to paddlers who have paddled abruptly from deep to shallow water. The critical depth can be found using the formula V= the square root of gD where: D = the water depth V = the velocity g = the acceleration due to gravity. Increased resistance as a boat enters shallow water can reach 150% of deep water resistance at the same speed. As the boat approaches this maximum the created waves change shape and at some point (depending on speed, hull length, and water depth) the bow wave system takes on a concave shape (radiating from the bow and when viewed from above) roughly at right angles to the direction of travel. The speed at this point is approximately Froude Number 0.99 with the wave traveling at the same speed as the boat and the wave-making effect concentrated in one wave. With additional power (creating a speed of approximately Froude Number 1.4), the wave pattern changes to one of diagonal waves only (convex when viewed from above radiating from the bow) and the resistance falls to levels below that in deep water. The difference seems so dramatic that marathon paddlers believe that the boats jump over their bow waves and begin traveling downhill. In reality, the wave-making phenomenon has altered and the production of energy-draining transverse waves ceases and is replaced by a diagonal wave system that drains away less energy. This lacks the drama of boats leaping over their bow waves but it avoids the difficulty of having to explain how the boat travels down the front of a wave it has yet to create. Changes in trim with increased velocity are symptoms of speed not causes of increased resistance. Efforts to counteract the changes may provide psychological benefit but no real benefit. Admiral Taylor suggests that such efforts may result in increased resistance. More often than not, what seems obvious in isolation reveals itself as erroneous in application. In this case Cliff did not know what he was talking about. There is no test data that I know of that suggests a bow shape that is particularly better than any other in shallow water. What is fine in deep water seems to work fine in shallow water. _________________ Cheers, John Winters

Thanks for that formula

I do a lot of paddling on water that is only 1-1.5m deep.

Pushing and maintaining a speed over 12.5kph is very hard work. 
The first time I raced on deep water, I was amazed at my finish time!