Mononobe-Okabe is used predominantly for analysis of retaining walls. Inherent in this approach is the deformation of the wall.
If the wall doesn't deform, this approach may not be applicable.
Now, if the concern is the mass of the water inside the swimming pool, that it is above ground, then you need to see work done on tanks retaining liquids for the appropriate approach.
There was an article concerning seismic design of tanks in a recent Structural Engineer magazine.
Good Luck