Theo I am fairly sure you are correct at least for some period of time until it has dried out and died. That is why when treating it with a contact herbicide you are told to spray it all , on logs , on the banks etc.

To be clear so all will understand, the reason I told trialsguy "If it was in the water it should die" is because the water had been treated with a systemic herbicide Sonar. Because of DW's toughness it is often necessary to spot treat the banks and left over spots with a contact herbicide even after the Sonar has worked. You have to stay after the stuff until it is gone for a while and if it pops back up immediately go after it again.