Falison, I am a geologist, NOT a heat pump expert. However, the conductivity of water is several orders of magnitude greater than soil materials. (Primarily, due to movement of the water, which is impossible for soil materials.)
Your question quickly becomes very complicated. Using water as the heat sink or the heat source in your closed-loop system is more efficient based on conductivity. However, it may become less efficient due to temperature differentials over the course of a full year cycle. The soil temperature will be close to the average annual surface temperature. The water temperature will slowly increase above the soil temperature during the summer. The water temperature will be highest when you are trying to dump heat during the summer. Likewise, the water temperature will be lower than the soil temperature during winter when you want a heat source.
Finally, I believe "efficiency" may NOT be the most important variable. I think(?) the reason people install closed-loop systems in existing water bodies is purely due to the installation and maintenance savings. Digging the trenches or excavations for the ground loops is a large percentage of the costs.
My advice would be to search the net for water-based heat pumps in general, and in your corner of the state.
You probably already know this info, but I included a link to some general heat pump discussion that also includes a section on surface water heat pumps.
https://www.wbdg.org/resources/geothermal-heat-pumpsGood luck on your project!
