Hi, a Brit living in the USA here.
I spent years and years around rivers in the UK and I can tell you one thing for sure - if you fall in a river which is deeper than your height (as Nicola may have done in bench area) and the river is around 40F in temperature, which it is at this time of year and you are wearing wellies / wellingtons (rubber boots ) (as Nicola was) - then, even if you are a very strong swimmer, it's just about impossible to get out.
The boots fill with water within seconds and act like lead weights on your feet. Also, once they have water in them, you cannot get them off under water.
That fact, coupled with the temperature of the watre, the depth of the water where she may have fallen in, and her other bulky clothing means that it is highly likely she will have drowned, probably within 30 seconds.
I fimly believe and feel that she did fall in, my only surprise thus far is that her body hasn't been found. The river Wyre flows at a pace of about 2 MPH in that stretch which is still fast enough to have near the estuary or out into the sea within a day. On this note I disagree stronly with Peter Faulding from SGI who believes "there is no way" she can have reached the estuary by now. But we will shall see. I still hope and pray she is alive.