In Ooblets, the term sea dangle means fishing, so you want to visit the docks to catch a variety of different items to use on your crops in your garden. Sea dangling is pretty straightforward, but you need a piece of bait to do it. You can start to do sea dangling when you first visit the docks in Badgetown after attempting to interact with the reconstitoot.
When you try to interact with the reconstitoot machine at the front of town next to the docks, Videon will approach you. He will break down that the machine accepts mystery cans that turn into the bait, which you can use to sea dangle. He will walk you over to the docks where you can interact with a fishing rod to try sea dangling. You will need a piece of bait to do this, and he does give you a basic bait for your first time. Sea dangling doers not require anything on your part and is a pretty quick activity.
You do need a piece of bait to sea dangle, and a quick way to obtain bait is to use the reconstitoot to turn any forms of trash or random items into a slurry. Some things may provide you with more slurry than others. For example, a wrapper you plucked from the sea will only give you one slurry, whereas a borgeous fungus yields three slurry. When you have 10 pieces of slurry in the reconstitoot you can make a piece of basic bait.