How to pass the jellyfish?

My advice, for now, would be not to bother. They don't always appear. You can get far enough into the sea levels to get some of the items without facing them if you're lucky. Just keep trying until you don't get them.

I tried sneaking under them, hiding down behind the 'sandbanks' until they pile up, and then kill the few that come down to attack you. It's easier not to get swarmed. If you find one of them coming behind you, just go upwards. I also recommend getting pink gloves and using acid rain. Same with the seahorses which usually comes after, just poke up with a fireball or acid rain.