I like a good bracing swim in Maine
waters . . . in July or August . . . on a sunny day.
Just Googling around a bit, I found an interesting graph showing water temps in Boothbay Harbor:
April (5C, or 41F) doesn't get more than a few degrees above the winter low temp (2.5C, or 36F), which seems to hit in late Feb. or early March.
June starts to improve (10C, or 50F). . . . .
The peak hits in August, at a balmy 17C, or 63F.
(My avatar pic is from Harpswell, near Stover's Cove. Love the whole coast of Maine, and can't wait to get back up there. . . .)