I think things like rubber rafts are already included as equipment in units such as infantry, which would allow them to cross a river and attack units on the other side; marines are just specially trained to do this, and therefore do not suffer the 25% penalty. In addition, engineers, even when under fire have been able to create makeshift bridges or repair existing ones in order for tanks to cross. Due to the size of the tiles, the tanks wouldn't be fighting units while crossing, but from a beachhead established by marines and infantry.