The steep sides make it unlikely to be a stock-watering hole. Purpose-built fishing pools associated with springs are known even now - for example at Poolspringe near Gamber Head, the possible site of Wonder 13, though the age of any that might be suitable has probably been lost. Roman Piscinae are found in Britain, and are often fed by springs, with elaborate piping systems to carry the water away*. Twenty foot would seem small on the basis of currently excavated examples (which are few), but not impossibly so, and, notably, most are a consistent depth across their whole span, ranging from 0.5-0.8m*. A short article on Roman fish-ponds can be found here.