Thanks to the Animal Crossing: New Horizons summer update, it's now possible to put on your Wet Suit and dive into the ocean in search of new sea creatures. In order to complete your Museum, here is a full list of these new shellfish.
This list will be updated continuously.
List of Sea Creatures
Above all, you have to learn to swim and dive.

Periode: N: May - Oct / S: Nov - Apr
Time: 4AM - 9PM
Shadow: Medium
Price: 1100

Period: N: June - Sept / S: Dec - Mar
Time: All day
Shadow: Medium
Price: 900 bells

Period: All year
Time: All day
Shadow: Large
Price: 500 bells

Period: All year
Time: All day
Shadow: Tiny
Price: 600 bells

Period: N: July - Oct / S: Jan - April
Time: 9 p.m. to 4 a.m. and 9 a.m. to 4 p.m.
Shadow: Large
Price: 12,000 bells

Period: All year
Time: All day
Shadow: Small
Price: 1000 bells

Period: March - June
Time: 9 p.m. - 4 a.m.
Shadow: Medium
Price: 1400 bells

Period: N: Nov - July / S: May ~ Oct
Time: All day
Shadow: Medium
Price: 500 bells

Period: N: Nov - Apr / S: May - Oct
Time: All day
Shadow: Large
Price: 4000 bells

Period: N: Jan - Mar, Nov - Dec / S:
Time: All day
Shadow: Large
Price: 8000 bells

Period: N: Mar - April / S: Sept - Oct
Time: All day
Shadow: Very large
Price: 12,000 bells

Period: N: Sept - Feb / S: Mar - Aug
Time: 4 p.m. - 9 a.m.
Shadow: Small
Price: 1400 bells

Period: N: Jun - Sept / S:
Time: 4 p.m. - 9 a.m.
Shadow: Small
Price: 3000 bells

Period: All year
Time: 4 p.m. - 9 a.m.
Shadow: Small
Price: 2500 bells

Period: All year
Time: All day
Shadow: Small
Price: 500 bells

Period: N: Oct - Feb / S: Apr - Aug
Time: All day
Shadow: Medium
Price: 5000 bells

Period: N: Apr - Aug / S: Oct - Feb
Time: All day
Shadow: Large
Price: 1500 bells

Period: N: May - Sept / S: Nov - Mar
Time: 4 p.m. - 9 a.m.
Shadow: Large
Price: 2000 bells

Period: N: Oct - Dec
Time: All day
Shadow: Large
Price: 4500 bells

Period: N: Sept - Feb / S: Mar ~ Aug
Time: All day
Shadow: Small
Price: 1100 bells

Found on the beach in ACNH
Period: All year
Time: All day
Shadow: Small
Price: 2800 bells

Period : October - December
Time : 9 p.m. - 4 a.m.
Shadow : Large
Price : 5000 bells

Period: All year
Time: All day
Shadow: Small
Price: 600 bells

Period: N: July - Sept / S: Jan - Mar
Time: 9 p.m. - 4 a.m.
Shadow: Medium
Price: 2500 bells

Period: N: Jul - Sept / S: Jan - Mar
Time: All day
Shadow: Medium
Price: 600 bells

Period: N: Jun - Dec / S: Dec - Jun
Time: All day
Shadow: Small
Price: 1500 bells

Period: N: Mar - June, Sept - Nov / S: May - Nov
Time: 4 p.m. - 9 a.m.
Shadow: Small
Price: 1800 bells

Period: N: Mar - May, Sept - Nov / S:
Time: All day
Shadow: Small
Price: 6000 bells

Period : All year
Time : 4 p.m. - 9 a.m.
Shadow : Small
Price : 300 bells

Period: N: June - Jan / S: Dec - July
Time: 4 p.m. - 9 a.m.
Shadow: Medium
Price: 2000 bells

Period: N: May - Sept / S: Nov - Jan
Time: All day
Shadow: Small
Price: 1700 bells

Period: N: May - Sept / S: Nov - Mar
Time: All day
Shadow: Large
Price: 15,000 bells

Period: All year
Time: All day
Shadow: Medium
Price: 1200 bells

Found by fishing in ACNH
Period: All year
Time: All day
Shadow: Medium
Price: 1200 bells

Period: N: Jan - Feb, Nov- Dec / S:
Time: 4 p.m. - 9 a.m.
Shadow: Large
Price: 10,000 bells

Period: N: Sept - Dec, Mar - May / S: Mar - June, Sept. ~ Nov.
Time: All day
Shadow: Small
Price: 1000 bells

Period: N: May - Aug / S: Nov - Feb
Time: 4 p.m. - 9 a.m.
Shadow: Large
Price: 10,000 bells

Period: August - September
Time: 4 p.m. - 9 a.m.
Shadow: Small
Price: 700 bells

Period: N: Oct - Jul / S: April - Jan
Time: All day
Shadow: Large
Price: 600 bells

With the summer update, new features and new furniture have landed on Animal Crossing Switch. While diving into the ocean, you will then be able to meet Pascal, who will give you Mermaid DIY Recipes. Here is the list.