With a fresh bottom, (minus silt, vegetation, leaves) this is the time to add a polymer sealant like soil floc. That is, if you are concerned that your bottom isn't holding water or your sides are not sealed properly. Even if you wanted to have faith that your pond has very little leaks, you will hopefully never have this chance again. You have 18" of water, is it enough to put a small flat bottom boat in and have someone pull you back and forth with ropes or propel yourself with a pole or oars? I would consider taking this chance to add a little 'insurance' that any seams in the bottom may have opened or the sides aren't sealed and consider a treatment of soil floc.
Then, when the rain comes (it has to since you are not in an arid zone) and snow comes it can help reseal the bottom. Downsides to this idea is cost, although you could go lighter on the application due to more ideal conditions in the bottom of your pond. The other downside is that the water is cold and the product works better when the water is warm. You could see how it goes this winter and in the spring when water temps pick up add it then (hopefully over the winter you will gain a couple more feet of water again?)
I can see how frustrating that is to look at the big hole and not see it fill up!
