There was a video I came across a while back that showed an average older guy jumping into a lake wearing full chainmail armor. The gambeson that is worn under armor actually acted as a flotation device, at least for the several minutes he was swimming around.
So I have no clue what impact armor of any type would be. If you think anything less than heavy armor should have no effect, try jumping into a lake wearing a winter coat and full backpack sometime and let me know how it works.
EDIT: forgot to add that plate mail was not 65 pounds, it was more likely to be around 40-50 pounds if it matters.
