Agree 100%, she could have been dumped in the river following an abduction. The river search is entirely justifiable, it's good that it has been done and should continue to be performed, as far in each direction as possible.
It's also possible she's still in the river somewhere and not yet been found, but the body will eventually float, if not entangled or held down. And if it's held down by something, deliberately weighted down, that's a 3rd party and a criminal act.
I don't discount the 'accidental drowning' theory, it needs to continue to be explored. I just also think there is a possibility she has been taken. And the fact that so early on in the investigation the police have entirely discounted it is premature and wrong IMO. There are many missing person cases that then turn into criminal cases, once more evidence comes to light. Lancs police should have kept that possibility open, if the focus changes, they're going to look a bit daft.