Bill is correct, in essence you are making a 12V battery.
There are two ways, actually. The first is as Bill described it, putting two 6V batteries in series (positive to negative, creating one “string” of 12V, and connecting the two positives and the two negatives of the strings together [the posts not connected to the other battery in the “string”]) the second is to parallel two 6V batteries together, twice (positive to positive, negative to negative) so you have two of these assemblies, and then connect those assemblies positive to negative to form the 12V “string”.
Use whichever way best fits your boat and how the batteries and wiring fit.
There are two types of fools...
One says this is old, and therefore good..
The other says this is new, and therefore better...