... and that'd be pronounced ...?
I've only heard the /slu/ pronunciation used in the States.
I grew up in the central Mississippi River valley in southern Illinois. The Mississippi has many such sloughs (slu). Some are dead-ended; others are like linked ponds. In the 1840s and 1850s a series of large floods changed the channel of the Mississippi south of present-day Ste. Genevieve, Missouri, north of present-day Chester, Illinois. Prior to the 1840s, what is today Kaskaskia Island was a somewhat long peninsula of land between the Mississippi on the west and the Kaskaskia or Okaw River on the east. Following the floods, the Mississippi cut across the peninsula and took over the channel of the Okaw forming two very large 90 degree turns. Separating today's Kaskaskia Island which is still a part of Illinois from the State of Missouri is a series of sloughs (slu-s), nearly all of which are navigable with small craft.
By the way, a famous English novel includes a slough in its narrative, the Slough of Despond. I don't know if Bunyan pronounced it slu or sluff. The novel is *Pilgrim's Progress* and the author was John Bunyan.