Decided to try these Odyssey batteries...
 

While at the lake on the 4th I noticed my batteries were getting a little weaker than I am comfortable with after long periods of coving out and listening to music; this was even after being on shore power overnight and the battery charger going into 'float' mode, e.g. fully charged. Not knowing the history of the batteries in the boat as I'd just purchased it last year, I decided to replace both the 'starting' battery and the deep cycle at the same time...decided to go with AGM technology. I've read a lot about these, 3 year full replacement warranty and real-world life of 8 - 12 years. A bit pricey but I felt it was worth it.
https://cimg8.ibsrv.net/gimg/www.off...06ab9935b4.jpg

I bought diehard platinum grp31 (which was made by odyssey) back in 14. One died last year. The other is still working as of now. They are a very good battery from all of my research. I would not hesitate to buy them again.