2. Temperature: The temperature at the bottom of the sea is usually lower than that in the swimming pool, which may make you feel cold.
3. Light: When swimming underwater, your sight will be limited, while in the swimming pool, you can clearly see the surrounding environment.
4. Current: The current at the bottom of the sea may be stronger than that in the swimming pool, which may affect your swimming speed and direction.
5. Breathing: When swimming at the bottom of the sea, you need to breathe through diving equipment, while in the swimming pool, you can breathe directly through your nose and mouth.
6. Sound: When swimming at the bottom of the sea, you will hear more sounds of marine life, while in the swimming pool, you may only hear your own breathing and underwater sounds.
7. Experience: Swimming underwater is a brand-new experience. You can see all kinds of strange marine life underwater, but the swimming pool can't provide such an experience.