The Little Britain star is raising money for charity by swimming 140 miles (225-kilometres) along England's River Thames from Gloucestershire to Big Ben in London.
Walliams has been battling to complete the challenge after contracting a severe stomach bug and struggling to keep food down.
But his spirits were lifted after his supermodel wife Stone visited him on Wednesday. The Dutch beauty was seen tearing up with pride and greeting Walliams with a big hug at the end of his third day.
Meanwhile, Brand was among the hundreds of well-wishers rooting for Walliams the following day.