Diving into a pool or river etc. while fasting does not invalidate one’s fast, provided that the water is not ingested via the nose or the throat. If water is in fact ingested while swimming, the fast is deemed invalid and the fasting individual must perform qaza for that day’s fast.
That being said, for a person who doesn't know how to swim properly, it is not appropriate to dive into a pool or river, etc., as there is a risk of ingesting water.