For many sailors who also spend considerable time on their boat when it is marina bound, here is a great solution to have the water tanks automatically topped up.
1. We use a little portable cooler
2. add a water cistern valve and some fittings.
3. Attach the whole contraption to the boat,
4. Connect up the water supply (via a timer to avoid any disasters)
And pronto: we have an automatic water top-up system when in the marina berth.
(see pictures below)