Having a mind like water is a concept adopted from martial arts where one responds only to necessary stimulus and returns back to the calm, but active-ready state. Having a mind like water is being at peace, while maintaining your "full potential." This means knowing a stimulus will come and also being present enough to keep the mind at ease.
We try to stay proactive vs. reactive, but reactions happen and sometimes they need to happen (think getting attacked, saving a glass from falling over, thinking fast in a business pickle). It's important that we know how to come back and that we are in control of our reactions as best we can be.
How does this relate to water? Imagine you drop a stone in a puddle. It will ripple, right? But it will also still. It will always return back to stillness. This is life, this is the intention, this is meditation.
How to Make Your Mind Like Water
To me, this metaphor goes deeper than this analogy, though. In Hawaiian Healing, the philosophy is "like heals like." As this is my practice, I took to my surroundings to learn and heal from. I have been spending daily time by a lake recently, using it in my meditations to learn from and mirror water. To make your mind like water, you must watch water. Fill up a glass, gaze into a pond, take a mindful shower, or swim in the ocean. Immerse yourself in the presence of water and watch openly.
Notice your emotions as you experience water. Feel into your physical state. Observe your mind. Reflect, like water does, after your water meditation and see if you learned anything about anything. Write it down.
I found that of all the elements, water feels the deepest, the most emotional, and the most sensitive. Each water meditation I did had me feeling submerged in my surroundings and very supple. I was in full acceptance of my fluctuating feelings and expansive ideas. I was deeply connected to my intuition.