You are attempting to address two unrelated concepts in parallel.
Water Tank holds freshwater. There is a pressure pump that pushes water to the faucets when a faucet is opened. The pressure pump shuts off when the pressure between the outbound side of the pump reaches a pre-configured level. When a tap is opened, the pressure drops and the pump kicks on.
An Accumulator tank CAN be installed on the freshwater system somewhere after the pump, but before the first T in the lines. It is pressurized and can be used to reduced short cycling of the pressure pump. They are nice to have, but not typically installed from the factory.
The sink and basin drains can be installed in three different ways.
1) Gravity: From basin drain to through-hull which dumps overboard*. This is common with the galley and head sinks.
2) Sump box: water drains from the basin (most commonly 'shower') into a small watertight box which contains a float switch and a pump. When the box fills, the float switch rises and the pump is actuated. The pump then discharges the water from the sump box overboard*.
3) Manual Discharge Pump": Some showers are plumbed from the shower drain directly to a pump. The pump is then plumbed to a through-hull which goes overboard*. This requires that you manually flip a switch to run the pump to drain the shower.
*Coastal boats are plumbed to discharge greywater overboard. Many, inland water (lake and river) boats have a grey water holding tank. Sink/basin discharge is sent to the greywater tank rather than overboard.