The Limestone Cave in Dave the Diver is found deep in the ocean, and you'll need to upgrade your gear a couple of times before you can venture that far down. This is just as well because there are plenty of angry fish more than happy to chase you away if you don't have anything powerful enough to take them out with.
It's not an area to venture into lightly, but if you follow the story, you're going to need to head there eventually; not only to stock up on fish for the sushi bar, but also to retrieve some parts for the quest, Where the Currents Flow. With that in mind, here's where to find the Limestone Cave, and how to survive once you get down there.
Dave the Diver Limestone Cave location
Everyone's map is slightly different so I can't pinpoint the exact location of where you'll find the Limestone Cave, but it's generally towards the centre of the map and you'll need to get down to a depth of around 70 metres. You'll know when you get there as you should notice large water currents pushing you in different directions, and there are often spikes on the walls which you can get pushed into if you're not careful. Many of the fish in this area are hostile, so you need to be on the lookout for these too.
The currents are easy to avoid if you time it right. They're not continuous so you can wait for them to stop and safely swim past before they start up again. The timing can be close, so just watch for a few cycles to get a feel for the lull.
If you're after quest items, look for yellow exclamation points above chests. If they happen to be in the direct path of a water current, you should have just enough time to open the chest before it pushes you away, and then you can swim back on the next lull and pick up the item.