As well as regular trips on the Woodside Ferry, I used to go to the I.O.M. most years with Mum & Dad on the steamers. The contrast between the diluted Oxtail Soup/gunge in the Mersey and the almost crystal clear water in Douglas Harbour was amazing. You could see the vessel's screws, rudder and even the bottom in places. Fascinating, when the visibility in the Mersey was about 0.05" !!
Because of the mud in the upper reaches that gets scoured down on the ebb, it will never be 100% clear. At least it's not just an open sewer anymore.