The hardest part for me was getting the iron for the bucket.
Load up Dwaggienite's world (-1618472320) and swim to coordinates 489, 34, 328. There is a sunken ship with a chest with a bunch of iron ingots in it. You need at least 3.
Go to the nearby island, knock down some trees, build a crafting table and craft a bucket. Arm your bucket and jump in the water. Swim up to a fish and press
This can be tricky. Sometimes you can still grab a fish even if you miss and there is water in the bucket, but more likely you'll need to go to land, dump out your water and try again.
Hope this helps!