I have (2) Deka Marine Master batteries. DC24. I checked their site and they don't make this one anymore. It's listed as 500 CCA whereas the new model is listed at 480.
My onboard charger charges at 13.6V. I see on their site it says it can boost to 14.4-14.6V. So, maybe they are being undercharged?
I haven't checked the fluid lately. I do have a battery shutoff that I use when it's not being used and for the past 3 winters it has been stored indoors heated. This year will be indoors unheated so I am debating leaving the batteries or pulling them out. I think they still perform pretty good all things considered.
We are on the verge of upgrading in the next year or two if we find the right model so I am not all that much in a rush to put new batteries in for next year.