Hi. I'm wondering if anyone has an opinion of using a tidal creek as the heat source for a geothermal heat pump. Many thanks!

No reason it couldn't be made to work. Tidal means excellent water movement twice a day - great for heat transfer. Tides also imply depth changes, salt water corrosion and bio-fouling issues. Design and install will need to accommodate those factors.

Freshwater biofouling consists mostly of algae and slime, not a big deal. Salt water biofouling involves thick encrustations of barnacles and shellfish...maybe you could farm raise mussels on the system's tubes, but you'd better make sure the heat will transfer through the fouling.

Excellent points, thank you. The issue of shellfish infestation is one I hadn't considered, and definitely a problem. I'll look into the difficulty of laying tubing below the subaqueous bottom and whether that would prevent the sort of problem you describe.

I would look at using copper or copper alloy pipes for a closed loop heat exchanger.  Also a water storage tank if the creek flow is insufficient at low tide.

Check into Slim Jim heat exchangers - those folks have probably dealt with this issue before - don't reinvent the wheel

Depth matters particularly in heating dominated climate as does body of government responsible for the water.
State DNR (or equiv.) may permit loops where Corps of Engineers may not.
Good luck,
Joe

Jonr - are you refering to the internal coil or are you saying put copper pipes in the tidal pool?

Copper and alloys have the advantage of resisting fouling (in the tidal pool), but I said "look at" because there could be other issues. The Slim Jim (titanium or aluminum) suggestion sounds like the best advice.