By all means if you have distilled water handy use it in your battery. Otherwise, whether you use distilled water or tap water, there are much more things at work shortening the life of your battery then the kind of water you put in it once or twice a year. The heat in the summer, cold in the winter, leaving your lights on, winching etc etc will do more to shorten the life of your battery. In fact many have never popped the top on their batteries to check fluid levels, usually just changing them out every 5-7 years. Even maintenance free batteries will lose water. In the Telecom business we do/did monthly maintenance on our large batteries, checking specific gravity and adding fluid monthly. Today, we use a lot of Gel batteries that are maintenance free and change them out like car batteries instead of maintaining them for 15-20 years where distill water could make a difference. Marine batteries are designed for cranking and deep cycling so you should be okay, just probably not worth the extra expense unless you got a deal.
Having said that, newer vehicles like the Chevy Silverado have smart charging systems that don't charge a "full" battery with a 14.5 volt equalize charge, but float it at a lower charge. This will mean less outgassing, and even lower maintenance needs. Never taking the battery below 50% charge and "smart charging it" and using distilled water might get you 10 years. I'd rather spend my $100 every 5 years for "piece of mind knowing it will start in the winter".
