Abalone

The abalone is a sea creature you catch while swimming in the sea (dive where you see bubbles). Look for a medium shadow. In the northern hemisphere look for it in Jun – Jan, from 4 PM – 9 AM. It can only appear after you have caught 20 sea creatures. Nook's Cranny pays 2,000 Bells.

Dive for a Medium shadow from late afternoon until morning, after 20 catches. It moves at a moderate pace, so follow it steadily; it sells for 2,000 Bells.