Depends on the model year. Some years had the head holding tank under the cockpit floor near the stern. Our 2007 had the integral head/holding tank, so the holding tank was at the floor of the head. Sealand Marine/RV toilet.
The 6 gallon water tank is in the compartment under the aft dinette seat. There is also room there for the battery charger and the galvanic isolater. A bit of room above the water heater for soft goods, like TP or paper towels.
You didn't ask, but the fresh water tank was under the v-berth in the early years of the 25 and moved back to the area where the head holding tank had been when C-Dory switched to the Sealand toilet with integral tank.
