Last night, I was at the same place. I did two photos of same object. After inspecting negative, I see nearly same spots on the water. I think, that spots are not on water in real eveview (exposure on the film was 4 minutes, my eyes are not possible to record this long time to one frame ). I think, that spots are from bubbles or lens flare, but I used lens shade. I want to scan other pictures from this lens and I will check spots. I will post results later.
Sorry, for my not so good English.