The original greenhouse is actually in a location with a very high water table (5' below grade) and it has provided an immense benefit to the greenhouse. The ground water provides a temperature and humidity buffer. If the water table is even higher than this (as it sounds yours might be) then you can take a two pronged approach. First berm the greenhouse above grade as much as possible. If I were in your situation I would put in a pond and use the excavation to create a big berm around the greenhouse. As long as the berm angle is 1:2 or shallower then you should get the same heat benefits. The other thing to do would be to add a french drain (or several) to reduce the tension on the foundation. This would drain the water around the foundation into the pond.