Think of it this way with hexes A, B (woods-stream) and C in a row. You fire from A. You can fire at anything in B, but the woods bit in B stops fire from any fire affecting C.
There are a few cases where you could not fire at an adjacent hex, like firing from a Pillbox through the NCA, but as a general rule firing at an adjacent hex is always allowed. The thing about a unit being in Crest on the opposite bank is that it still is in hex B. It in real life might be closer to C than A within the B hex, but it still is in B.
If B was simple woods then anywhere in B is vulnerable to A but anything in C is not. Think of the B-C side as a magical barrier to A's fire. Now let B be woods-stream again. The only difference in real life is that there is some "open ground" in the form of the stream. Indeed A would be "shooting them in the back" if it fired on a unit on a Crest at the B-C side.
So you have A, A-B hexside, possible Crest at A-B side, IN stream, unit in Crest at B-C side, B-C hexside and finally C. IE the Crest status is before the magical B-C hexside barrier.