Water is after all reasonably transparent, and would likely let most of the light travel through itself, so it's not like you can just focus all that energy on the surface of the water.
This interestingly enough would be one of the major flaws. Not only would it let most of the energy through, providing negligible resistance, it would also magnify the intensity of the beam.
How would it do that? It would absorb some of the energy (how much would be adjustable by suspension concentration). Even if there were some sort of lens effect in play, lasers aren't really that easily focused since they are already coherent beams of light. Moreover the water would cause scattering and dispersion of the beam like any substance that radiation passes through. and it definitely doesn't magically add power to the beam...
The most important effect though would be how much of the heating power could the water conduct or transfer away from the point where the laser hits the inner surface under the water layer. If the water layer is able to absorb and disperse the beam enough, it might be weak enough not to be able to heat the inner surface enough to melt or vaporize it, especially when the water is there cooling the surface.
With respect to armor, the best materials that would be practical are either sodium, beryllium, or magnesium. Magnesium is out for obvious reasons (tends to catch fire easily). Sodium reacts violently with water, so wouldn't be effective anywhere there is even more than a little water (read: anywhere but a desert). Beryllium has an enourmous specific heat (the amount of energy required to heat one gram of that substance by one degree celsius or kelvin, in this case 1.83 J/gxC) and also a very high melting point (around 1278 degrees celsius). It is non-magnetic, and is also very light-weight. It is also very corrosion resistant, so it would only have to be replaced after direct combat damage. The greatest downside to it is that it is relatively rare.
Not to mention beryllium is
pretty darn harmful (I think even more than asbestos or depleted uranium).
The next best thing would be aluminum (0.91 J/gxC), but we've already gone through how that would do in my previous post.
Well, in terms of
volumetric heat capacity water is number one, which defines the physical dimensions (or thickness) of the layer you need against the laser. Water also has very very high specific heat, so even though it melts and boils at lower temperature than beryllium, it would still likely be more useful for space ship armour... as it can also be used as a heat sink and water supply as well as food supply (you can grow algae in some sections).
Certainly, beryllium has high melting point but it's also a lot less practical than building a lightweight dual-layered structure and filling it with water, and putting critical components within the water layer protection The inner and outer surface materials should obviously be something solid, but as far as
Obviously stuff like sensors, thrusters and weaponry that needs to be on the outside would still be vulnerable but structural integrity of manned sections would be protected along with life support and reactors and other critical components.
