Trying to find a British children’s picture book about a dog

“I’m looking for a late 1970’s(?) UK children’s picture book featuring a cartoonish dog protagonist. it is illustrated on every page but I think it is for adults to read to young children. The story is a ‘one-day-in-the-life’ structure about a very intelligent dog… Perhaps anthropomorphic. At the end, the dog wants to go swimming but is told only humans are allowed, so he disguises himself in his human suit and wins the competition. Illustrations are pen-and-ink with watercolor. I read it in a library copy around the year 2000, but I think it was at least 10 years old and maybe more by then. Could you help identify it?”