If you're having trouble catching a Dave the Diver jellyfish, you're not alone. When you first start out, you're likely to come across these sea creatures in various locations, but any attempt to catch them with a harpoon or gun is met with failure; clearly, their jelly-like exteriors somehow make them immune to physical damage.
You'll have a similar problem with tuna when encountering them, and there is a different way to catch seahorses, too. Those are problems for chapter two though, so if you'd rather just stick to the fishing woes of the early game, here's how to catch jellyfish in Dave the Diver.
Dave the Diver jellyfish: How to catch them
During the first chapter, you'll get a few days' warning that a group of enthusiasts are going to show up at your sushi restaurant for a Jellyfish Party, so you need to make sure you're prepared and stocked up.
Jellyfish are all but immune to physical damage, which isn't great news when all you've got to your name is a harpoon and a gun. Luckily, you can find temporary weapons inside special crates underwater, and you'll soon get the ability to craft your own which you can keep. The one I use to take out jellyfish is the basic Net Gun. As the name suggests, this fires a net at your target, and in the case of the jellyfish, renders it defenseless so you can swim on over and retrieve it from the net by pressing the space bar. It seems that other weapons that either stun or put fish to sleep will also work, though I haven't tried these myself.
As the weapons that spawn from chests are random, there's a little luck involved in unlocking the Net Gun. However, once you find three of them inside weapon crates, you'll unlock the ability to craft a permanent one for yourself. The one downside to this gun is that you can only shoot it three times before you need to find more ammo, but it can catch several fish simultaneously if you manage to sneak up on an unsuspecting school.