You can also check out chain auto stores like Auto Zone, etc., as they sometimes have good prices. Regardless of where you buy, when you are looking around you should also check the date code systems because you want to buy the newest battery
they have stocked, not the oldest.
Be sure you are buying a marine DEEP CYCLE (aka trolling) battery, not a marine starting battery!
I have carried in a volt meter and checked them, comparing dates and voltages, and the newer ones indeed had higher voltages. Aside from overcharge damage, freezing, and stuf like that, depth of discharge before recharge, and then the number of full discharge/recharge cycles will define how long a battery will last. Out on the tail end of this stuf is age, so starting with a young one will help, because self-discharge during storage leads to permanent sulphation.
Here's a long excerpt from The DeepCycle Battery FAQ
Lead-acid batteries are perishable and sulfate in storage due to their natural self discharge, especially in temperatures above 80° F (26.7° C). Please see Section 16 for more information on sulfation.
Determining the "freshness" of a battery is sometimes difficult. Unless it has been periodically recharged or is "dry charged" (shipped without electrolyte), NEVER buy a wet Standard (Sb/Sb) or Low Maintenance (Sb/Ca) battery that is MORE than three months old or a wet "Maintenance Free" (Ca/Ca) battery that is MORE than six months old. Dry charged batteries are shipped without electrolyte, but usually have "sell by" dates of one to three years. Depending on the temperature, AGM (Ca/Ca) and Gel Cell (Ca/Ca) batteries that can be stored six to 18 months before the State-of-Charge drops below 80%. Please see Section 16. for more information on sulfation. Dealers will place their older batteries in storage racks so they will sell first and they do not have to maintain them. The fresher batteries can be found in the rear of the battery rack or in a storage room. For a wet battery, the date of formation is often stamped on the case or printed on a sticker. If at all possible, have a new battery tested, and recharged if necessary, before the battery leaves the store. This can save a lot of time and frustration if the new battery is sulfated or has a manufacturing defect.
Some of the manufacturer's formation date coding techniques are as follows:
7.6.1. Delphi (ACDelco) and some Sears DieHard
Dates are stamped on the cover near one post. The first number is the year. The second character is the month A-M, skipping I. The last two characters indicate geographic
areas. For example, 0BN3=2000 February.
ISBA Delco Date Code
[Source: Interstate Batteries]
Douglas uses the letters of their name to indicate the year of manufacture and the digits 1-12 for the month, D=1994 O=1995 U=1996 G=1997 L=1998 A=1999 S=2000. For example, S02=2000 Feb.
7.6.3. East Penn, Exide (Champion), Johnson Controls Inc., Interstate, Chrysler (Mopar) and some Sears DieHard)
Usually on a sticker or hot-stamped on the side of the case. A=January, B=February, and the letter I is skipped. The number next to the letter is the year of shipment. For example, B0=Feb 2000.
ISBA Date Code Sticker ISBA Interstate Date Code
[Source: Interstate Batteries]
7.6.4. Exide (some Sears non-Gold DieHards)
The fourth or fifth character is the month. The following numeric character is the year. A-M skipping I. For example, RO8B0B=February 2000.
ISBA Exide Date Code
[Source: Interstate Batteries]
The first character is the year. The following three numeric characters are the days of the year. For example, 3123=3 May 2003.
The date code on the negative post is stamped as the battery comes off of the finishing line, ready to ship out or go into stock. The code that is stamped is usually one month ahead. Therefore, a battery that comes out in March will carry an April date code. The code on the positive post is the manufacturing date that indicates when the battery was physically built but before the addition of any electrolyte. The letter is the month (A=Jan, B=Feb, C=March, etc.) and the number is the actual date. So "K26" means that the battery was ready for electrolyte filling and the first forming charge was on November 26th. Since the negative post shows A2 (January 2002), the manufacturing year has to be 2001.
The activation date is found on an orange sticker on the shipping carton or email Concorde Customer Service with the serial number of the battery.
7.6.8. Rolls and Surrette
The four digit date code represents the day of the week (first digit), week of the year (middle two digits) and the year (last digit). For example, April 4, 2003 would have 4143 as a date code. The date code is stamped into the front edge of the cover of the battery.
7.6.9. U.S. Battery
The three digit date code represents the year (first digit), month (middle letter) and the plant code (last digit). For example, April, 2003 would have 3Dx as a date code. The date code is stamped into the positive terminal of the battery when it is formed. The characters burned into the case are the production run. For example A270N.
7.6.10. Other Date Coding Methods
The four digit date code could represent the week of the year (first two digits) and the year (last two digits). For example, November 1, 2006 would have 4406 as a date code. The four digit date code could also represent the month of the year (first two digits) and the year (last two digits). For example, November 1, 2006 would have 1106 as a date code. The six digit date code could represent the month of the year (first two digits), day of the year (middle two digits), and the year (last two digits), or any other combination. For example, November 1, 2006 would have 110106 or 011106 as a date code. The date code is usually stamped into battery or printed on a sticker attached to the battery.
If you cannot determine the date code, ask the dealer or contact the distributor or manufacturer. Because of permanent sulfation due to self-discharge, a fresher battery is definitely better and does matter.