If you google the east river, you will find out that this time of the year it is very cold.
https://www.answers.com/Q/What_is_the_current_water_temperature_of_the_East_River_in_NYC
That would just about stop your heart.
And in other links, you will find that the current is quite strong a treacherous.
No way he would let go of that raft.
It’s not technically a river as I recall from my manhattanite years
More a tidal estuary