[Spaceships] Pool as a fuel tank

[adimar]

I'm currently designing a small luxury space yacht for a high ranking executive.
The engine design uses water as fuel, and one of the cool features that came to mind is using one of the fuel tanks as a swimming pool (Not really an original, i think this was written from one of the space odyssey books)
My intent is that the pool will be a standard fuel tank (with some minor internal customizations) and when half the fuel was used it would be used as a pool.
How would you stat this, in terms of fuel capacity?

Here is the design for reference purposes.
Peregrine space yacht (TL10, SM6)
This is a winged, streamlined design.
Capable of both atmospheric flight and excellent space performance.
And best of all is quite cheap to operate since it uses water as it's reaction mass.
A perfect craft for the busy executive or a crack team of operatives.
0.12g , 72mps, 30 tons of water.

Front
1) Adv. Metalic laminate
2-5) Habitat (Passengers: 1 Luxury cabin, 2x Standard cabins)
6,Core) Fuel tank (Water, doubles as a swimming pool)
The entire front section is mounted on an axis an spins to create artificial gravity.

Middle
1) Adv. Metalic laminate
2-5) Fuel tank (Water)
6) Control room
Rear
1) Adv. Metalic laminate
2-5) Fusion rocket (High thrust, Water: 0.03 per engine)
6) Engine room.
Core) Habitat (Crew: 1 Bunkroom)