What can wavy patterned sand tell about the Fluid that formed it?
I don't know any literature specifically on the mechanism of dune formation, but the physics subfield which studies sand is "granular media", a subfield of statistical condensed matter physics.
Granular media has only been active for a decade or two. The focus of granular flows is on the surface flow in sand or other particulate aggregates, which differs from fluid flow in that only the top-layers in sand move. But as far as I can make out, the detailed physics of granular flow isn't the significant factor here. The only important thing is that, unlike a fluid, sand will make a stable equilibrium when the surface is at a nonzero slope less than a critical slope $S_c$. This phenomenon leads to a memory effect, called hysteresis. If you make an indentation in sand, so that you set up a sloping surface, it will remain stable, so long as the slops is not too big. This allows patterns in sand which are sculpted at a finite wind velocity to remain stable even when the wind dies away, so that the sand equilibrates to a shape which is stable over the average speed of the wind.
Mechanism
The force responsible for forming the dunes is the same force which pulls up your umbrella on a windy day. When a wind is blowing in a certain direction, where the surface is curved downward, the wind will deflect down, creating a negative pressure which pulls the umbrella up, and where the surface is curved upward, the wind will deflect upward, creating positive pressure which pushes an indentation. When impacting a soft hysteretic surface like sand, the wind pressure will dig holes in places which curve upward, and it will move the sand locally to settle in places which curve downwards. The result of this is an instability which forms a pattern of dunes whose peaks and troughs are perpendicular to the wind velocity, and whose characteristic wavenumber is determined by the average velocity of the flow, the density of the fluid, and the height of the fluid.
I assume the mechanism of forming sand dunes in wind is the same, but for air-sculpted dunes, you will get additional complications from turbulence. The tops of sand dunes can be have further instabilities which go on to deform the shape periodically along the peak of the dunes.
It is also possible that a similar instability is responsible for setting up water waves in deep ocean in response to a constant wind.
Estimating the instability wavenumber
First, the equilbrium law. When sand is placed at a constant pressure, it will statistically settle down so that the surface is horizontal. If the sand is jostled and allowed to resettle, the surface of the sand will be flat.
When the same sand is exposed to a slowly spatially varying hydrostatic pressure P(x) on the bottom of the water, the sand will settle locally so that the force profile is the same as flat at any point. This means that the internal sand-o-static pressure from the changing sand depth needs to be enough to cancel out the horizontal variations of pressure. This happens when the depth of the depression produced is proportional to the applied pressure, or when
$$ P(x) = -(\rho_S - \rho_W) g h(x) $$
This equation describes a pressure equilibrium in a circumstances where the pressure is varying from place to place. $\rho_S$ is the density of sand, $\rho_W$ is the (necessarily smaller) density of water, $g$ is the acceleration of gravity, and $h(x)$ is the height of the sand on the bottom.
Now supposing that the water is flowing with velocity v, and the bottom h(x) is sinusoidally varying
$$ h(x) = H \cos(kx)$$
then the water velocity must curve with the bottom (note that this is not easy to solve exactly, the sand violates no slip to some extent, since the first layer of sand will move along with the water a little, but the essential fluid flow follows the pattern of the sand). The estimate for the pressure differential established by a flow with velocity v is the centripetal acceleration for a given mass of fluid to go round the bends.
The height of the bends is H over a distance 1/k, so that the inverse radius of curvature is $Hk^2$ (this is correct for more than dimensionally, it is expanding cos in a second order Taylor series). This gives a centripetal acceleration
$$ a_c = H k^2 v^2 $$
Where $v$ is the constant horizontal fluid velocity. This gets multiplied the the fluid density $\rho_W$ times the height of the part of the fluid that curves along the bends, which is given by the decay of the vertical components of the velocity, which is by a Laplace equation, and so the height is approximately 1/k. The result for the pressure is the maximum pressure, modulated by the height variations, to account for where the pressure is least and greatest, according to the curve on the bottom, or
$$ P(x) = - H k \rho_W v^2 cos(kx) $$
This goes into the equilibrium condition
$$ g(\rho_S-\rho_W) h(x) = - P(x) = k \rho_W v^2 h(x) $$
Which is solved by $h=0$, reflecting the obvious fact that if there are no sinusoidal variations, the pressure is constant, and this is an equilibrium. The condition for stable equilibrium is that the net linear coefficient in the equation determining h is positive. This can be understood physically--- a small perturbation of h at wavenumber k will produce either a smaller pressure than required to sustain the perturbation, or a greater pressure that will deepen the perturbation. If the pressure is more than is required, it will deepen the perturbation, and if the pressure is less, the perturbation will die away.
So the stability condition is found by equating the coefficients of the left and right side of the equation above for h:
$$ {g\over v^2} ({\rho_S\over\rho_W} - 1) = k $$
This determines the critical k for instability. For MKS units, $g=10$, $v=.5$, sand twice as massive as water, and assuming the water is deeper than the wavelength predicted by this relation, k is 40, corresponding to a wavelength
$$ \lambda = {2\pi \over 40}\approx .15 $$
For larger k, there is an instability, while smaller k are stable. The critical k will be the wavelength of the dunes that are formed. So from the approximately 5cm wavelength of the waves in the sand, I would say that the typical velocity of the water at that spot is approximately 20cm/s, perpendicular to the wavefronts.
This is only slightly better than an order of magnitude dimensional analysis estimate, but since the picture is given, it assures one that the correction factors are not too big, at worst case a factor of 2 or 3. A more precise calculation is certainly possible.
Note that the sand is constantly shifted by the moving water, and this is important, since otherwise it would not find the correct dynamical equilibrium with the waves.
Restoring equilibrium
In the approximate description above, the loss of equilibrium condition is from the first order in H condition. This means that once the wave starts to grow, its growth will not be checked.
The condition which checks the growth is the critical slope for sand. Once the critical slope is exceeded, the sand will flow downhill, washing out both higher order in k perturbations, and leading to a sawtooth profile for the equilibrium height, with a slope equal to the critical slope for sand flow.
The relationship is not unique, because bedform patterns typically evolve even with a fixed flow field. Some pattern-scale statistics are robust, for example in the images shown the bedform crestline orientation is very likely nearly perpendicular to the flow direction on average (in space-time). Other features are much less robust: For example in many cases of migrating bedforms, average height and wavelength increase through time as smaller bedforms merge together.
Reading some of the other answers, I offer two clarifications:
technically a "dune" is a particular type of bedform. (In dunes height scales with flow thickness, they are a "depth-limited" bedform).
granular physics has been studied for much longer than decades, e.g. The Physics of Blown Sand and Desert Dunes (1941), mostly under the label "sediment transport" or "morphodynamics", and mostly by civil engineers (see e.g. Albert Einstein's son Hans )