Water cooling, as mentioned, is more bulky than air cooling solutions and would pose alot of issues for a laptop.
Also, it can be expensive for decent water cooling solutions.The cheap water cooling solutions are no better than decent air cooling and just take up more space for no real benefit.
This is why many who invest in them carry them over from build to build. With a laptop this would be a issue as well. As a one size fits all would be extremely difficult to pull off.
With water cooling, you need a pump that can move the water efficiently through the loop. How well it cools depends on surface area of the radiator, and the airflow through it. In other words, the bigger the radiator and more water it holds the better it cools.
How big will it need to be to trump the air coling in place with out adding alot of weight and size to the laptop? And by that I mean a balanced tradeoff that makes it worth the effort.
And you must remember, that water cooling is not a solution to everything. You still need some airflow throughout the chassis.
Since laptops are not really upgradeable, and fall behind in technology rather quickly, this would be a sizeable investment for a relatively short lifespan, being tied to one machine. You can't just upgrade parts as needed. What you have is what you get for its lifetime.
I see this going down the other path, which is already in the works. Faster more efficient parts that use far less power. Which equates to lower heat.
this is always the case with technology as it progresses but mobile parts are becoming a larger focus.