[TimT]

Not sure how closely the water table level matches the tides...

But that pool looks only 3-4 feet deep... Not sure if there is a deep end

Say the tide and water table elevation match inch for inch, the still would be only be a small amount of horizontal pressure on the walls......

Then there is buoyancy... you could float that pool right out of the ground..

The contactor should partially fill the pool to make horizontal pressure and bouyancy close to zero...