I was thinking the same thing - are the witnesses absolutely sure they saw her? From a distance it is easy to think you see someone you know, but when you get closer you realise you are totally mistaken. The fact she was not wearing a bobble hat is worrying, if this is what the witness saw (I am not sure if they did see this - I am just wondering).
Also the phone on the bench issue, I too sometimes put my phone down next to me when handsfree. I almost left it behind on one occasion in an enclosed shopping area - I got up, walked a few yards and then realised I hadn't got my phone so turned and retrieved it. So it could have been left behind accidentally and maybe Nicola did not go in the water in that area, if in fact she went in the water at all.