Your colon is very good at extracting water; if you can hold it in then yes in theory it could remove some of the water. However in reality when you have diarrhoea your colon is inflamed and irritated during and you can’t really stop it happening. You’d have to overcome the cramping and the body trying to push the contents outwards.

The most likely outcome of trying to hold it in is it gets worse and you get an explosive and even less controlled expulsion.