From what I've seen a little "bulging" is normal on most batteries. However more starts to become a concern.
Of course a little/normal/more are all very subjective so...
I guess a good question would be are you actively cruising, living on the hook, at the dock
? Making a passage
soon? If you rely on them daily, and have gotten 6 years that's not bad. How are their temps when charging
? What's the worst case scenario if they fail? Will the acid be contained? I suppose it could start a fire but that might be a little far fetched and usually happens when old batteries are trying to be charged.
Just my random thoughts I'm by no means a battery
expert or even that knowledgeable compared to many around here.