Since you will have hard time to find a waterbed in the mattress store showroom, you will spend most of your time doing your research and shopping online. There are several well-established waterbed manufacturers and a few newcomers to this category so compare specifications deliberately and read plenty of customer reviews.
Waterbed warranties vary quite a bid depending on the type of bladder, material used and manufacturer. Most of waterbed warranties start with the short non-prorated period (2-5 years) followed by longer prorated period, with overall warranty duration between 7 to 20 years.
Make sure you ask your store about exchange/return options and exchange/return costs. Read the fine print of the exchange/return policy before you place an order. Some retailers charge delivery and old mattress removal fees that can significantly add to the final amount.