I disagree. A lead acid battery should last at least 20 years under optimal conditions. I have been working with marine batteries (and other types)for about 10 years now. That's actually what I do for a living. I have been dispatched to several power plants, nuclear and otherwise, to assist with their power (and battery) problems. I have been hired to help Sony DVD manufacturing, Cornell University, Mitsubishi Electric Company, US Dept. of the Treasury, FEMA, NASA, and others. But don't take my word for it, here is an excerpt from Exide's FAQ page:
Certainly, vibration has a detrimental effect on the life of the battery, but not to that extent. If you use a car/boat battery the way it was designed, it will last 5-6 years. If you leave it for 6 months without recharging it, it's toast.