You can take a taxi and pay to get in but they only sell so many wristbands and it is first come first serve so you may not get one that way. I don't think it is much cheaper that way either.
We have done this a few times and we are going again next month. I think it is about how much you like waterparks. My family and I love them, we go to them all over our area all summer so we appreciate how much better this one is than most. If you are not a big fan, you may find that it is not worth the price. I recommend everyone trying it atleast once if they can afford it and they enjoy this type of thing.
We usually book at room at the Comfort Suites next door, this gives you full access to the Atlantis. Depending on the rate and how many people you have in your party it can save you quite a bit, if there are only 2 after taxed it usually ends up being about the same.
Just a few notes about the property. They do not accept cash outside. If you want to eat or drink outside or you want to rent a locker you have to have a credit card with you. Lockers are first come first serve. The food outside is similar to amusement park quality and prices...DO NOT LEAVE FOOD UNATTENDED even for a second, there will be a feeding frenzy with the birds.
If you don't have kids with you on the way back from Atlantis stop at Senior Frogs and get a Goombay Smash in a yard glass...YUM!
Have fun, I love Nassau.