One definition of recoup is indeed "to regain." As Webster's Third New International Dictionary (1986) puts it:
3: to get back: REGAIN ["an attempt to recoup his fortunes" —W.J. Ghent]
But another—and according to Webster's, earlier—definition has a different tendency:
2a: to make good (as expenses, losses) ["this is largely recouped to states from taxes" —John Kemp] : to make up for ["recouped their losses"]
The relevant version or portion of definition 2a for our purposes is "to make up for": You may lose water or lose time, but you can make up for those losses literally (by collecting water to replace the lost quantity) or figuratively (by making better use of your time going forward). In both instances, "recoup lost X" is not redundant or self-cancelling, any more than "compensate for lost X" is.
Incidentally, the "John Kemp" cited in the quotation associated with the first part of definition 2a is, I believe, the guy much better known to longtime U.S. football fans and Congress watchers as "Jack Kemp."