much of eastern poland, northeastern poland, parts of latvia, lithuania, belorussia (belarus), much of ukraine and the whole area around st. petersburg near finland are comprised of swamps and lowland marshes. combine this with a lot of snow (usually around one metric fuckton if i remember correctly) in the winter equals a veritable quagmire of mud in many places during the spring months, where it also happens to rain a lot.

casualty numbers were high, but i don't think they were necessarily high because of the swamps, they were high because of poor training, poor equipment malfunctioning, a repressive political system that was rigidly and dogmatically opposed to efficient tactics and leadership, a dogged and determined enemy, partisan poles, lats and baltic peoples, desertion and of course the obligatory police executions and arrests.

i have never read anything on whether or not they had special training to cross swamps or marshes, though i'm sure certain small tactical units did have specialised training. especially in areas where they were infiltrating cities to spread propoganda, ie near Warsaw, they would have chosen experienced locals and natives who didn't have 'training' per se but who knew the land well and could navigate and maneuver through the dark and through otherwise difficult or impassable terrain without being detected.