The point about DCs or torpedoes in shallow water is very true.
Bombs, cluster, HE or DC, are the only viable weapon for shallow water ops. SWO, by its very nature, will be limiting on the submarine in both speed and obviously depth.
The main reason for a submarine, invariably a conventional, on SWO would be for SF insertion. Now the kill radius of a DC against swimmers is rather better than against submarines!
In many waters a sub-surface submarine may be visible by day especially against a pale sandy bottom.
Helos, with DCs, are therefore an ideal shallow water ASW weapon, a high speed pursuit ship may spot a submerged submarine but will be much less able to relocate and deliver an accurate attack.