Sussex Police assisted coastguard units from Selsey and Littlehampton in the search for a person seen entering the sea at Bognor Regis on Saturday (December 22).
According to an RNLI Selsey spokesman, the inshore lifeboat (ILB) was launched at 8.47pm and made best speed to the area, where a shore line search of the immediate area was conducted, although nothing was seen.
The spokesman added: "At 9.12pm the ILB helmsman requested the all-weather lifeboat to assist with the search. The weather on scene was wind westerly force 2-3 sea state smooth with good visibility.
"At 9.24pm the all-weather lifeboat (ALB) launched and headed to Bognor. The ILB had been joined by the police helicopter."
As no one was found as a result of the search, the spokesman said the police helicopter was tasked to another incident, but both lifeboats and coastguard units continued searching until they were stood down at 10.30pm.
"The lifeboats both returned to station at 11pm with the ILB being recovered straight and the ALB recovered at 1.45am after waiting for the tide to fall," the spokesman confirmed.
"Both boats were washed down and made ready for service."