Tim,
I had 25+ dead trees that were in my pond for 8 - 10 years. When the pond was drained down for repairs - I chain sawed most of them down.
That was the hardest wood I have cut in my life! I don’t know why – maybe have something to do with oxygen depletion? Those trees would have lasted many more years had I not whacked them down.
Go for it.
Gator