You start at the top left. Swim down. There are air pockets to breath with. You must avoid the floating jellyfish. Other enemies are needed too.
You may run into the purple jellyfish, these can be activated and start to follow you.
There can be secret tunnels where extra health + bonus stuff can be found, these could be hidden by seaweed. A clam can be seen at the bottom that blows up bubbles. you will be blow up a little when you swim into the stream of bubbles. If you touch the clam it will snap shut at you and hurt you. Fish can swim on paths back and forth, so you have to swim around them. Electric (or sharp, dangerous) seaweed can hang from the top and bottom to make you swimcarefully.
When you get to the top again, the way is blocked by a large jellyfish... it is a mother jellyfish who wants her children, indicated by a card witha little purple jellyfish, with a x6 (meaning she wants all 6). If you havent yet found all 6 purple jellyfish, you must go find them. When you have all 6, you will be able to activeate her action.. at which point the babies go to her and they all swim away (to the right)
You can now swim to the right... to blisters submarine. There are other wrecked ships and junk there