Russians walk on water?

[Der Kuenstler]

Hey guys - I abandoned part of downtown Stalingrad during my second scenario try, thinking that no Russians could get over there where there was no bridge or ferry. But then I found this:



You see a Russian unit crossing the water. When I click on the hex it says water - no ferry or bridge - so what gives?



Also, when I click on my infantry they are followed by a (-) sign and the letter "W" - anybody know what this means?



While I'm asking, how come the Russians crossing the river earlier are isolated, but the Russians in a stack upriver here are not isolated? Don't see a bridge in either place. Thanks in advance!