The reason a gell cell battery lasts so long is because there not constructed like a typical lead acid battery. Conventional batteries have plates in them that dissinegrate from discharge/charging and vibration which causes the particals to build up in the bottom and eventually causes a short on one of the plates. Another battery killer is discharging the battery below a certain percentage of total capacity. Once this happens a few times, The battery will never regain full capacity in amp/hours no matter how long you charge them. As for Marine batteries, They are designed to take that kind of abuse and still be able to charge to capacity. The downside to this though is typically they don't have the cranking power of a normal car battery (most). If you took the same size class battery from marine and auto the auto battery will have more cca then the deep cycle battery.