I'd love to find this cartoon again as it brings back some childhood memories.
My memories of it are vague as I believe I only partially recorded it on a now destroyed VHS tape or it was just a short mini cartoon. It featured a man (potentially a pirate/explorer) getting off a red and/or blue ship at the docks and finding his way into a cave. The cave itself was a dull blue or purple hue. In the cave there is mountains of gold - or at some point he finds mountains of gold. I distinctly remember some sort of small river in the cave that leads to a pitch black tunnel. In the archway of the tunnel there's a dragon head at the top which prevents him from entering further by blowing fire at him. At some point a lime-green two headed dragon appears as well as some sort of underwater shark-dragon monster that tries to eat the mans boat. I'm unsure if he made it out with or without treasure, but he does eventually make it back to his boat on the dock. I don't remember much else aside from these things. (I'm more than happy to do some quick drawings of some things to better describe what I remember seeing if needed).
I'm in the UK and it must've aired sometime during the early to mid 2000s although I have a sneaky suspicion it was re-aired from an earlier date (possibly 70s or 80s) as I remember the animation style looking outdated for its time or it was just very low budget.