bond plexiglass to concrete

how smooth and flat are the surfaces around the window openings you left in the concrete? your most important problem is sealing the windows to those surfaces; once sealed, the panes can be held in place with a couple of pins at the bottom if necessary (i.e., if the sealant lets it slump down, which really isn't likely).

what is the inside surface of the pond going to look like? epdm liner? colored sealer of some type? natural concrete? that will affect your choice of sealant unless you don't much care what it looks like from the top.

if you're going with the native concrete surface, i would suggest a good aquarium-grade silicone sealant. put a couple of mirror supports in along the bottom to hold the pane up until the silicone cures completely

Silicone will not stick to bare concrete.

You need to paint and seal the concrete surface first. Once the concrete has been sealed then you can apply the silicone. Typically holes are drilled through the plexiglass and stainless steel tap cons are used to hold the plexi to the concrete. The silicone is used to fill in the area between the plexi and the concrete and is sandwiched in place. A washer placed between the plexi and the concrete will provide the spacer to keep the silicone from being squished out.

Andy