Yes the freshwater tank is for both. Yes, fill with the hose. I turn hose on full blast and overflow, so any algae contaminants will float out the top. May want to add a tad of bleach for algae buildup.
The old Seafarers have a 10 gallon tank a little behind the cabin door, that feeds the rear shower and the sink in the cabin. Later models have the water tank at the rear of the boat and the filler on the starboard side of the livewell next to the plastic rod holders .