The first step to making a body of water is deciding what materials to use. A very innovative way to create the look of water without getting anything wet would be to use a floral design product called Wonder Water. This is a clear gel which thickens and hardens, yet looks exactly like water. You could add dye to it in order to create a more dramatic color, even possibly shape the top with a tool as it hardens if you need to create a choppy oceanic setting!
Another good item to consider is called Friendly Plastic. This product comes in tubs and consists of small, soft plastic pellets that soften when placed in warm water. You can then clump them together and literally mold whatever waterscape you desire. The plastic can be dyed or painted, and the best part is that you can reuse the material over and over again simply by placing it back in warm water to soften! This can become an excellent advantage if you make an error in shaping your “water”, or simply want to save money by being able to use the plastic later for other purposes.
These creations can be embellished in many ways. You can add drops of glue to the surface to enhance details and define ripples in the water. A glue gun will create perfectly sized beads that can serve as bubbles for the edges of your surf. Hair gel can also be added in or smoothed over the top of your creation and will be soft and pliable enough to allow you to animate a constantly rippling water surface.
Perhaps these options do not work well with the style of your particular film. Another nice trick to try is to use opaque, patterned glass (similar to patio table glass) and blue reflective paper underneath. When shooting your frames, simply move the paper underneath the glass to create reflective ripples on the sparkly surface of the glass. The effect can be quite dazzling.
A surprisingly realistic look can be achieved by using only a blue sheet (or paper) and plastic wrap. Place long sheets of kitchen plastic wrap over your sheet or paper and then carefully crush and scrunch the surface to create movement. The plastic wrap can lift and peak in some places, creating nice whitecaps and waves. If you keep the movement going between frames, the overall effect can be fantastic. For added shape, do not be afraid to use scotch tape behind some of the plastic wrap in order to help you achieve the look that you desire.
Any of these techniques can be manipulated in post-production, but you may find that these in-camera effects will require no extra tweaking after the frames have been shot. Experiment with several of these methods until you find the one that works for your film, and then dive in!