At 8 feet there will be considerable water pressure. With concrete block walls, they will need to be tied in well with steel and poured concrete.

At sea level air's pressure is 14.7 pounds per square inch (PSI).

Each foot of water creates water pressure of 0.43 PSI

0.43 x 8 feet is 3.44 PSI + air pressure 14.7 PSI ( one atmosphere) would equal

18.14 PSI at the 8 foot depth.

Just for the fun of it, how many pounds of pressure would be exerted on one concrete block under 8 feet of water that is 8" x 16"? 128 square inches x 18.14 PSI or 2,355.2 lbs. of pressure?