Impossible to say with any certainty. Could be nothing, however, it could be a bad cell, corroded terminal, etc... Are the batteries disconnected from the boat or from each other or both (i.e., isolated)?
The resting voltage of two isolated batteries is meaningless. Check each cell with a hydrometer to ensure they are all good. You can find the proper range of readings anywhere on the internet
. The specific gravity of a bad cell will be obvious.
The above is a lot easier than doing a load test which might be preferable except they are 8Ds...
The biggest problem you may have is moving this size battery
if one is bad.