Snail larvae. The water was loaded with these. Probably the periwinkle Littorina littorea. An invasive species introduced to Canada from Europe in the 1800. It is abundant on rocky shores throughout New England. I am not sure how far south they are found.
Here are 2 pictures:
Side view of the shell. the blurring at the top right of the image is due to the movement of cilia along the edge of their feeding apparatus called a velum.
End on view:
the picture is fuzzy but you can see the extended velum and the cilia along the edge.
And a video: