Been reading these threads and just wanted to offer an anecdote to those who don't believe Willow wouldn't have followed NB into the water...
I owned a springer spaniel who loved the water, she'd be in there at any chance she got (like most springers). Despite this, when myself and some friends were wild swimming out in the lake district, my springer was strangely reluctant to join us. Something about seeing us in the water freaked her out. Even calling her in with us, she was keen to get back onto dry land. Springers are very sensitive dogs, and for me it's not beyond the realm of possibility that Willow would have been confused/upset about seeing a person in the water when that is not something she usually experiences.