Think about the problem before you start anything else. A water bottle needs to hold water but must also look appealing and intrigue the consumer enough to want to buy it. It also needs to be durable enough to withstand some minor scuffs but cheap enough to produce. All these problems should be considered as you design.
Start with some paper and a pencil and sketch. Sketch anything you think of when you think of water. Do not consider anything that might hold you back. Use elements of water in your design (for example, you could design a water bottle shaped like a water droplet).
Design the cap and label. Is the cap plain white or colored? Remember, production may cost more if it is colored. Are you going to design the label or are you hiring someone to do it? If you hire someone, make sure he receives the proper dimensions of the bottle so he knows what to do.
Choose a material for the outside of your water bottle. Will you use glass or plastic? Keep in mind the production costs, which can vary greatly depending on the material.
Consider any textures you may add to the exterior of the bottle. Will it be rough or smooth? Also consider the size of the overall bottle. Keep in mind that water bottles are sold by the ounce or liter, so the bottle needs to be designed to hold the average water amount.
Find a local manufacturer and ask if it would be willing to give you a prototype of its design. (Keep in mind that it may not have the time or resources.) If it can't, ask if it would be willing to assess your design and estimate an average cost of production. If you are serious about using your design as a product to sell to consumers, you will need this information before you begin to search for a company that will hire you.