too lose belly fat you should swim with a pulse over or mear the aerobicthershold for at least 60 minutes. Similarly if you run or bike.

the disadvantage of swimming compared to biking or running is that at the finish, you are "cold". When you finish running or biking you are "warm" or "overheated". While your body is overheated you loose appetite. on contrary, when swimming exactly in the moment after training when  you areboth  mentally and physically weaken you are also "cold" and thereby you have not problem to ingest big amounts

more info on link below
https://www.sportsci.org/news/compeat/fat.html

so recommendation… swim long and avoid walking by a fat food restaurant after and ingest some fruit or protein direct after training