The East River has super-strong currents (tides, actually) as well as ships, so it’s something you very well may die doing, but there are lots of ways to kill yourself. Welcome to adult life.
This should be treated the way skiing in the back/slack-country is handled in Utah.
The last paragraph of the red sign sums it up:
“You are responsible for your own actions, your own rescue, the cost of your rescue, and waive all claims for injury.”