Our Tolly's current batt bank consists of four LA, group 31, deep cycles hooked up in parallel. They are from Batteries Plus chain of stores and manufactured by East Penn Battery Company.
Purchased early summer 2010 makes them 8 yrs old. I've added distilled water as needed; always distilled! And, do not let them drop below 50% charge before 100% recharge again topped off. These batts are not same quality as some other rather expensive deep cycle batts. However - they cost me only right at $400. So, divided by 8 yrs. = $50 per year for batt power aboard.
Are they as strong now as when new 8 years ago - Well, no! But, do they still work well enough for our general needs - Well, yes. Do I expect much more out of them - Well, maybe this summer or next will be their end. Will I purchase the expensive deep cycle batts or again buy similar at low price from Batteries Plus - We shall see!
Bottom line I'm trying to get across here: Boat's batt bank can be outfitted with considerably affordable deep cycle batts that by keeping maintenance up correctly and when utilized/re-charged correctly can make boating pretty easy to enjoy for years on end before batt bank needs new batteries.