From the Watchdog FAQ
Q. Do you need a Basement Watchdog battery?Seen lots of complaints about them failing in 3-4 years. So based on that they are no better.
A. You can use any deep cycle marine battery, as long as it is not maintenance-free. However, there are several advantages to using a Basement Watchdog battery. The Watchdog battery accommodates the system a fluid sensor which will sound an alarm when the battery needs water a very important feature, since a dry battery will not perform. In addition, Basement Watchdog batteries run longer per charge, and last years longer. A typical marine battery will last 2-4 years, while the Watchdog battery will last approximately 5-7 years. Basement Watchdog batteries are shipped dry (without acid) so they stay fresh on the shelf indefinitely, while wet batteries have a short shelf life.