Cows can swim well, they could have taken a boat out and lassoed her with those SSPCA loops. I don't think they tried.
I don't know much about boats but I'm not sure how feasible it would be to lasso an animal weighing nearly 3/4 of a tonne from one?
We work with cattle on land, cattle that know us and know the land, and it can take a heck of a lot of doing sometimes to get a recalcitrant one where you want her. They are particularly difficult to handle when there aren't any other bovines about. I'd have got a couple of coos, penned them onshore and waited for her to come up to them - but I don't know if she could in fact get out of the water where she was.
I'm more puzzled about how she came to escape in the first place. Does anyone know how that happened?