People who want to burn more calories while swimming should use things like kick boards. They also work the muscles in their limbs by using things like paddles, flippers, and life buoys, which are all water toys. 
swimming equipment 

Short-distance swimming: Many people swim slowly, which burns more calories than fast short-distance swimming. But if you swim fast all the time, your physical strength will be used too quickly. Is it possible to do two slow tours, then two fast tours?

Swim slowly

Professional swimmers train by breaking their swimming into four parts, with 15 to 30 seconds of rest in between each part. Swim one round-trip first, then two round-trips, then two round-trips, then two round-trips, and then one round-trip. At the end of each section, you should swim quickly.

Professional swimmers

Do everything you can while swimming. Do everything you can to keep your heart rate around 80% of your maximum heart rate at all times. In order to meet this standard, smart glasses will record the swimmer's heart rate and show it in front of their eyes in real time.