Always use a GFI outlet to plug-in your walk-in tub! This is an extremely important step to prevent any possible accidental electrocution in the future. Make sure that your outlet is always connected to it's own breaker, as your existing wiring to your bathroom probably wasn't sized up for the amperage load you're going to be pulling.
Double check that the wiring you've purchased is suited for the conditions in which you'll be using it. It's always better to over buy and get a lower gauge wire, than to cheap out and get a wire that's too small for the intended purpose. The last thing you want is for your house to burn down around you while you're trying to enjoy a soothing bath.