Tell me this. Have you already got your aquarium set up, or are you talking about the first time water?
If you are talking about inital set up, it is a great way to start the cycle! I encourage you to try it. I would avoid using live sand though, take that from the ocean too. If you can take a trip down toward the Keys to get it, I would go that route! If you dont, go for the off shore sand bars or dive for it. The farther from the beach you are, the better off you will be. I did something similar when I started my tank. I am in North FL though, so our water is a bit different.
If you are talking about using ocean water for water changes, I advice against it. There are too many variables in ocean water. You can get infections (in your fish) by using ocean water. You can also increase your nitrates becaues there are so many living critters in the ocean water that will just not make it in the tank. This could lead to diatoms, cyanobacteria, green bubble algae, the list goes on and on. I would seriously consider the negitive effects of using ocean water for water changes. It would however be a great source of food for your fish, if you could figure out how to kill off the bad bacteria and keep the photoplankton alive in a tank.