The water should easily hold the plexiglas in place against the silicone sealant, making that sealant act as a gasket to prevent any leaks. If there is a problem, it looks to me like the bowing out of the plexiglas due to the water would be the problem. Neither plywood nor plexiglas is really very stiff - both will balloon out with the water pressure. But, how much they balloon out may be insignificant. I know I wouldn't put any holes in the plexiglas for bolts. I think I would glue a strip of wood all around the plexiglas, so it can't move up or down, or sideways.
Also, remember that both wood and plexiglas will creep with a load on them, which might slowly add more stress to all of the joints. That would encourage me to glue vertical 2 x 4 or 2 x 3 reinforcements on all of the plywood panels.