A tide mill is a very simple idea. Rather than use a mill-pond to store
water to drive a water-wheel, you create a pond and let the tide
fill it with water for you.
This is the plain version of the photo below.
The routine was:
- open the sluice gate at low tide
- wait for the tide to come in and fill the pond
- close the sluice, as the tide falls outside the higher water level in the pond drives the mill
- at low tide, open the sluice and start the process all over again
No fuel costs, carbon neutral, 6th century (500s) technology, maybe earlier.