@Oaktree55 and others on river and tide observations. Good points.
I live next to a river too. It is about the same size and depth as the Wyre is at this village. The differences in the rise and fall within any period, even 12-18 hours, can be quite staggering after earlier heavy rain. When it rises and falls has much to do with other precedent weather conditions. Sometimes the surrounding land, from the source upstream right to the basin, will be of just the right condition that it will soak all rainwater up like a sponge. Then it might release it quickly, or it might do so over a longer period of time. I have seen ours fill up within 6 hours, but sometimes it has taken 36 hours for heavier rain upstream to raise levels.
Even a local person, will not always be in tune with their river. Those skills are being eroded. A generic "its shallow and not fast flowing" is not evidence when you might spend just 10 seconds twice a day crossing the road bridge in a car, in the depth of winter. It might have looked low and have a calm flow several mornings in a row, but on this particular morning it might have been slightly higher, deeper and faster in the morning, but by afternoon much more subdued. The police will be onto the weather and will have info resources, but I am not convinced they have the funding necessary to confirm accurate river levels and water flow in the timescale of their reporting this week.
Nor do I think Peter Faulding had all the information on the water flows in that exact spot. His work looks technically brilliant, but it is not infallible. A lot of what he has otherwise said does make sense though.
People do not completely vanish. If they die at a spot, normally some trace is available. In my country, it was always blood that gave some closure. Even then ransoms could be paid and bodies returned.
People can remove themselves and leave no trace. Likewise, an organised individual can abduct another person and leave no trace. One of these is the likely outcome here. People can disappear with no trace if it is planned right.