A couple considerations:
First, no castle is completely surround by water [aka - a moat]. The few exceptions or close situations to this are cities like Dagbor which has very strong looking stone bridges. Thus, players will never be in a situation where they can build and then hold or destroy the only bridge into their cities.
Second, rivers and streams and creeks vary in depth and width with every map, since no concept like this was put into production - meaning that it would either be way too long on some rivers or way too short on others, also, I am not sure how it would work into the whole pathfinding of units. However, in the scenario editor, I know that there is a capability to turn any object into a bridge, whether it be a fallen tree, or a vine, or a lengthy twig, or a real bridge! So, I guess there are many options for versitality, though I am not sure how this would help in the implementation of such an, er, ambitious unit [as a river crossing siege weapon].
Here is a link to a thread in which moats were discussed [keep in mind it is an older string of posts]: