I am sure you know the batteries need to be deep cycle and there about three choices in types of batteries out there. The first and cheapest is the standard lead-acid deep cycle battery. Nothing really wrong with these and they will normally last 3 to 5 years if you keep water in them and store properly. The second is AGM which has a glass mat between the plates. Advantage is maybe a bit longer life, not as easily damaged in rough water, will not leak acid if damaged or turned on side, and does not need water addition. Disadvantage is cost. Third and most costly option is spiral plate like an Optima. People that have them swear by them but I can't justify the cost. Same advantages as AGM plus I think they have more capacity for the same size battery.
I looked around and wound up buying Cabelas AGM batteries on sale. I have had them a year and they still hold full charge so far. I would recommend a new digital on board charger if you don't have one no matter what batteries you get. It will charge the batteries and then condition them once charged so you can leave it plugged in and it will have your batteries ready when you are plus they will last longer.
If I had gone conventional batteries, it looked like Academy Sports had the best price when their Exide batteries were on sale.