During Elizabethan England, "letting out of ponds" was a crime:
In like sort in the word felony are many grievous crimes contained, as [...], stealing of whatsoever cattle, robbing by the high way, upon the sea, or of dwelling houses, letting out of ponds, cutting of purses, stealing of deer by night, counterfeits of coin, evidences charters, and writings, and divers other needless to be remembered.
What does "letting out of ponds" mean?
Where ponds were used to keep live freshwater fish (a kind of fish farming), letting the water out enabled poaching a large number at once. They tended not to be little round ponds, but artificial lakes created by damming streams. Rich landowners would raise carp or other fish and poaching gangs would make breaches in the dams, 'letting out' the water (allowing it to escape). Preventing or investigating such crimes was one of the tasks of a gamekeeper or estate steward.
Sir George Savile, in addition to his deer, had fish ponds which poaching gangs were prepared to drain to get at the fish. At least twice Savile had a dam cut in the period 1715—17 in order to steal the carp from the artificial ponds the dams created. In February 1717 discovering that Savile's 'little stew pond' had been robbed, [Thomas] Smith [Sir George's Chief Steward] rode with a band of estate servants to a village well known to be a haunt of poachers. They had a mixed reception as they began searching the houses, one of the suspects' wives 'exceeded the scolds of Billinsgate' (sic) and bestowed blows on the keeper's man. The searches were all fruitless, but within a few weeks Smith and other estate servants had a very clear picture of the poaching gang responsible. The leader of the gang was a man called Widdison, who chiefly operated with two accomplices, Beesly and Alwood. Widdison seems to have occupied the same place in Smith's nightmares which Richard Roe had occupied in William Thynne's two generations earlier. Four months after the stew-pond robbery Widdison was in gaol serving a three-month sentence for poaching in the fishponds of John Digby of Mansfield Woodhouse, a neighbouring estate. A year earlier he had been convicted of poaching deer in Savile's park but Savile had reprieved him from gaol on condition of good behaviour, an act of clemency both Savile and his steward bitterly regretted because of his subsequent crimes. These they had good reason to believe included breaking dams and stealing fish on at least two occasions, including the stew pond incident of the previous February.
Stewards, Lords and People: The Estate Steward and His World in Later Stuart England (DR Hainsworth, Cambridge University Press, 1992)
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.
