In this interview from last October
The Leonard Lopate Show: Highest Duty - WNYC Capt. Sullenberger describes his decision-making process. With only a minute to go before impact he "spent several seconds choosing my words carefully, which is a lot of time given how time-compressed our experience was", and said "This is the Captain, brace for impact." He says "I chose
not to announce we were landing on water, because I didn't have time to give the full explanation, and the situation I wanted to avoid was where the passengers might be reaching under their seats for life vests, maybe even attempting to put them on, instead of bracing for landing, when they might have been injured during the touchdown."
The relevant passage starts 25:45 into the interview.