I get you, however, if there is one deep spot below the bench she could hypothetically drown in... Where is the body?
There is
drone footage showing them walking the boat up a decent stretch of the river with confidence, only one bend down stream, both in the middle and standing in the water by the bank. If there was a hole of depression by the bench, the search area to find her body would either be quite small and obvious. Or the body would have to rise out of the hole, then float down river where it would be very obvious and prone to getting stuck on something in a shallow slow moving stretch.
To get out to sea a body would have to traverse down a shallow river for miles, go through several hairpin bends, fall into a much wider and low current old millpond, come out the other side and work its way through even more miles of river. All the while not being spotted by the significant air and water search teams that were in scene very quickly after her disappearence. As well as all the volunteers walking the banks or locals keeping an eye out in the downstream riverside walks and the towns/villages the river goes through.
Libby on the other hand was put into a treacherous and notoriously fast moving river at night and swept a much shorter less complicated distance out to the Humber (which hardly can be called a river, it's seven miles wide and has a vicious tide.) Nobody was looking in the water for a week after she vanished, by which point she was almost certainly already where they eventually found her. It's a very different set of parameters and amazingly lucky she was found at all.
If this happened a lot further down The Wyre then I'd still be all for this theory. It can get a lot deeper and more treacherous, but that is miles and miles downstream of here.