Super Mario 64 on N64 - The Secret Aquarium Star

The aquarium is located in front of Course 3, in a small square hole in the wall, fairly high up (a backward somersault makes it easy to reach). You find yourself in a large room completely filled with water. Swim straight ahead to the first red coin and turn left to find a second red coin.

Keep going around the room at this height to find the next 2 red coins, then dive down to the floor to find the fifth one.

The last 3 coins are on the floor in the 3 remaining corners. So swim around the room calmly (but not too calmly, or you'll drown) and collect these 3 coins to make the star appear right in the center of the floor.