Although in modern usage pond is often applied to any small body of still water, historically it refers to an artificial body of water built for a specific purpose, hence mill pond, horse pond, fish pond, and indeed curling pond.
The OED etymology links pond and pound, in the sense of an area for enclosure or confinement, cf. dog pound or impoundment. The most common and important of these in medieval times would have been fishponds, i.e. dammed up areas where fish could be concentrated for the local food supply.
To quote Historic England's description of the fishpond complex at North Kelsey Grange:
The tradition of constructing and using fishponds in England began during the medieval period and peaked in the 12th century. They were largely built by the wealthy sectors of society with monastic institutions and royal residences often having large and complex fishponds. The difficulties of obtaining fresh meat in the winter and the value placed on fish as a food source and for status may have been factors which favoured the development of fishponds and which made them so valuable. The practice of constructing fishponds declined after the Dissolution of the Monasteries in the 16th century although in some areas it continued into the 17th century.
With regards to let, the OED entry notes a very specific sense of let out:
7a. To allow the escape of (confined fluid)…
1832 E. Bulwer-Lytton Eugene Aram I.i.v.84 Mr. Walter..wants to consult you about letting the water from the great pond.
This is the same sense in which bloodletting is understood.
So, considering a pond was something built and maintained at some expense, and which therefore was probably owned by wealthy landowner or large institution like a monastery, releasing water from it would have been viewed askance. As Michael Harvey observes, lowering the water level in a pond would make it easier to poach the fish within, and thus letting of ponds would fall in line with poaching from deer parks and the other crimes enumerated.