I like to talk about this topic because I have experience.
I have swum in the sea twice. The first time was in Sanya in 2010, the second time was in Weihai in 2017.
In the winter of 2010, I went to Hainan Island with a tour group. When I was at the beach of Yalong bay in Sanya city, many people swam in the sea. I was not prepared for swimming in advance, so I didn't have a swimsuit. But it didn't matter, I just took off my coat and swam in the sea. Maybe tens of minutes later, I went back to the beach. Other people took a fresh bath after swimming in the sea, but I didn't because I didn't want to spend ten RMB. I just go around the beach and bask in the sun until my body dries. Of course, I took a shower when I got to the hotel.
I used to buy a swimming pool membership in a five-star hotel, but I didn't swim too much there because I was lazy.
The difference between the two kinds of swimming is feeling. When you swim in a pool, you don't fear it because you know that it's very safe: the water in the pool is not deep, the pool is not so big, and safe guards are watching you, so you won't encounter any danger.
Swimming in the sea is a very different feeling. The sea is endless, the waves are strong, the water is so deep, so you have to be very careful to watch out. Even if you know it is safe, you still have an unknown fear.