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 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.