Listen to the recommendation for a pair of GC2s in series. I did a lot of work on performance cruising boats (read retired keel boat sloops) before the depression shut us down.
There are VERY few real deep cycle 12V batteries available. A "Marine" battery is not one. What is sold as a Marine battery is an automotive battery with a small (if any) compromise. (That compromise is usually in the warranty.)
Then there is the problem that without jumping through flaming hoops, a pair of jars in parallel will never provide capacity X2. You will be lucky if you can get 1.75 but more than likely it will be about 1.5 and I can tell you why, but I have other things to do this morning. Just remember that Lead/Acid batteries do not play nicely in parallel.
Matt