Walmart and 3 months old: Aren't they still under warranty? I have exchanged failed batteries at Walmart. Walmart will test whatever you return to be sure it is failed. Your batteries are not quite failed yet, but it is an option worth considering for the future.
1) Are you sure that exposure to river water caused the effects you now see? Yes, water could get in through a vented plug, but how much really got in? Moreover, you have flushed the cells. Did you use the smart charger to test capacity just before the incident? I would not assume they were any better before the incident than what you see now.
2) As mvs sarma points out, the cells were partially discharged when they got flooded, so using full strength battery acid to flush may give too high a specific gravity when fully charged. Slow charging to a fully charged state, then flushing with acid of the correct specific gravity would address that concern.
3) As I recall, the main difference between deep cycle batteries and starting batteries is in the plate construction, not the electrolyte. I would recommend using the electrolyte recommended by the manufacturer and/or dealer.
John
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