The boat left North Berwick and headed to the east end of Milsey Bay after concerns were raised by members of the public.
A search was carried out in the area an the crew were able to identify the swimmer safe and well on the beach 20 minutes later.
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Earlier that day, the same boat was launched to assist a broken down powerboat with two people on board off Seacliff beach to the east of North Berwick.
Once on scene the lifeboat was able to established a tow and brought the casualty vessel back to North Berwick.
The report came in around 5.20pm on Saturday.
In a statement on social media, the RNLI remind people that in any coastal emergency, they should dial 999 and ask for the Coastguard.