There are a lot of fencing companies around, many of which do glass pool fences, and they all have their preferred suppliers for glass and fittings.

Is there deck joist on the underside of the decking, which leaves me to believe open air space, my question is more about drainage under the deck. Is there a French drain or catch basins to divert the water from collecting and causing algae growth or mosquito haven areas... Or even an increase of wood rot.

Yes. Correct drainage is important in all areas of the garden. Air circulation and good drainage under decks like these are a must.