Often used in series for more than 6 volts. At least in the applications i've seen, though i'm a long ways from a sea and lakes may be different.

yes, because the underlying cell voltage is much lower than that. Many 6 volt batteries still exist because they are used in series to make the desired voltage. They are easier to move around and transport when they aren't build as all the cells in one unit.

….As part of a bank of batteries producing a higher voltage. I haven’t seen any 6v equipment manufactured for any engine based equipment larger than a couple of KW for a very, very long time (mid 1960s). Lower voltage means higher amperage means more weight, more cost, more heat, more failures. Objectively, the world would be a significantly better place if we had fully transitioned to 24v or higher (up to 48v anyway) much sooner than later.

Interestingly, micro-miniaturization has reversed the trend of higher voltage = higher efficiency, at least for computing.