Just seems like ground water no? Wasn’t there last year?
For the hell of it you could try digging a hole and you’ll find out quickly if it’s ground water.
I was on the Cape for a week in June 2021. Weather was great, but we probably lucked out a bit. July into September is best there.
If I ever got lucky, I’d get a second place there in the Brewster to Orleans area. I love the bay side.