My boat is also a 2006 Mariln. Last year one of my batteries failed, another was not charging properly. In addition my charger was acting up so I decided to replace all four original Deka batteries and the original charger. Looking back the charger may have caused the batteries to fail or vise/versa.
As far as the voltage goes, I would be a little concerned. My stbd side is reads a little different than my port side (forget which side powers the electronics, gauges, etc), but this is only at idle. As soon as I increase the rpms the voltage increases (as the alternators produce more amperage) and pretty much reads the same 13.5-14.2 volts if memory serves me correctly. I am not certian of the exact numbers, but I am certian that the voltage reads higher as the rpms increase, not the other way around.
I would start with the simple things, check all connections on batteries, levels in batteries, connections at engines, etc. If nothing is obvious, I would then check each battery individually for voltage. I suspect after six years your batteries may bee showing signs of their age (mine certainly did). I decided to replace all four as I head offshore for ovrenight canyon trips and getting jumped is not an easy option for us.
Keep us posted.