Is there a clear water MineCraft mod that will work for SMP servers?

Solution 1:

OptiFine has, among many other improvements, a clear water option (under Video Options -> Details). This is a pure client modification and works just as fine in SMP. (Note that this is not compatible with MCPatcher, which is however not needed since OptiFine also includes HD texture support).

Solution 2:

As far as I know, texture packs work entirely locally (note that this might change). This means that you'll be able to use it on a SMP server without any problems. The only thing that you have to do is find (or make) a texture pack that does this for you and install it.

Solution 3:

Try the CrystalClearWaters mod: I'm not sure if it's SMP-friendly, but if you install it locally it should only affect the rendering on your own client.

Solution 4:

If you want to increase your visibility underwater legitimately, recently (in MineCraft 1.6), the Respiration enchantment on helmets also increases the ability to see underwater.

Respiration can go up to 3 levels, and the stronger the enchantment, the better your underwater vision.

Screenshot taken 6 blocks below the water's surface on the highest brightness setting, with and without Respiration enchantment

For more details, visit the MineCraft Wiki page on Enchanting.

Solution 5:

If you want to see more clearly in water without needing to resort to mods, a Potion of Night Vision will drastically help.

The following forum post contains comparison shots of with/without the potion effect (and the water is even more clear if you are not swimming in it, but instead looking down into the water from the surface):

Minecraft Forum: Potion of Nightvision - underwater?