I used to watch this movie as a child but have no idea what it is called:

It's about a young boy who flees a house and runs down to the nearby river/lake and jumps in. Once he goes underwater, he becomes a cartoon and all the fish and sea creatures around him are also in cartoon. They can talk as well. I can remember that he becomes friends specifically with a sword fish. On the surface everyone is looking for him.

Can anyone help??

It's the 'Water Babies'! I loved it as a kid, even though it kind of freaked me out a little!
Here's a clip of it YouTube - The Water Babies - High Cockalorum

Thank you so so so much! Not knowing has been killing me!