Rocky1,

Recognize that I'm in Texas and certainly do not have similar climate conditions to you. It may not be practical for your situation, but this has worked very well for me so I offer it for your consideration:

In addition to shallow water structure, there can be significant benefits associated with deep water structure. You have a large area 15-18 feet deep. Much of that area may very well go largely unused by your fish without the presence of any structure. That is a lot of area to just "waste".

My solution was to add underwater "islands" in the deep areas rising up to within a couple of feet of the surface. These islands were created during construction with pilled up dirt and brush on top. That may not be an option for you and may not even be desired if swimming in that same area is planned.

However, it works. The largest BG ever taken from my ponds was from one of those islands. The largest HSB was also taken from an island in deep structure. That approach I feel, has made productive water out of what otherwise would have been unproductive water.