No, the jet ski boast was not a joke. It was a lie.
Joke is defined by Merriam-Webster as something said or done to provoke laughter while a lie is an untrue statement made with the intention to deceive.
It was a lie, planned with his campaign staff, which he used in almost all of his 2016 campaign rallies, complete with flag-kissing. There was the INTENTION to deceive.