Construct a tank 5m tall with 3m x 6m base and fill it with water. Draw a line .5m below the surface. On one of the 3m wide walls of this tank, draw a matrix of .20mx20.m squares, with the matrix being 10 squares wide and 25 squares high (2m wide by 5m high). In the center of each square, there is
an infared port that acts as a link between the cubes and the wall.

Now take some cubes (slightly denser than water) that have computer controlled latching mechanisms on four sides (height and width axes), and an infared interface on one of the two remaining sides. Put handles on the sixth side. Latch them into traditional tetris shapes. Teams of four SCUBA divers must slide the pieces down the wall, rotating them as desired, and release them so as to form a solid line of cubes across one row of the matrix. If a solid line is formed, the computer signals via the infared interface to latch said cubes together and unlatch them from all other cubes, thus allowing the referee diver to pull them out. He injects air into the cubes' bladders, and they float to the top.

Once the pieces arrive at the top, they are reassembled into more tetris shapes, and then chucked back into the pool at a set interval. Scoring is like that of regular tetris. Although the pieces don't fall faster at higher levels, they are thrown into the pool at a higher rate, forcing the divers to work quicker.

This would be unbelievably expensive to create, let alone maintain, but definitely fun.

I think that the shapes don't need to be removed, just be stacked as neatly as possible without reaching the surface.
A tank with transparent sides would allow spectators.