Colombo is perceived by most tourists, first of all, as a large city that can be visited for the sake of attractions and excursions, or simply as a transshipment transport hub on the way from south to north or back. Nevertheless, there is a whole resort area with good beaches and tourist infrastructure, and some tourists stay in it for their entire vacation.
The area with beaches in Colombo is called Mount Lavinia (Mount Lavinia or Mt Lavinia). It stretches along the coast to the south of the city, 20 kilometers from the center (Mount Lavinia beach on the map).
The beach in Mount Lavinia is sandy, wide and quite suitable for swimming and relaxing on the shore. The main line of the southern railway from Colombo runs along the coast here, and no matter which hotel you stay at (with rare exceptions), you will have to cross it, but by and large this is not a problem. But right on the beach there is a chain of restaurants and inexpensive cafes, there are some beach activities and equipment for rent, and even a dive center.
Although administratively Mount Lavinia is a suburb of Colombo, it is considered an urban area, and you will not even feel the difference and will not see the border between the center and this area. Here, just like in the center, there are hotels, travel agencies, cafes and restaurants on the beach, currency exchange, banks, and Colombo city transport. But it's much quieter and calmer here. Having settled in this area, you can get to the center by tuk-tuks, taxis, or local buses and commuter trains. Buses run along the main road Tissamaharama Rd, also known as Galle Rd or A2 highway, very often, cost mere rupees. It takes quite a long time to get to the center due to traffic jams, 30-60 minutes, but you can use commuter trains. There are two stations in this area at once: Mount Lavinia and Dehiwala. The train to the center to Colmbo fort station and others run from 5 am to 10 pm approximately every 15-30 minutes, the fare costs from 15 rupees. And from here, even during the day, you can go to other beaches and resorts on the southwestern coast of Sri Lanka: Beruvela, Bentota, Ambalangoda, Hikkaduwa, Galle.
But Mount Lavinia is not the only beach in Colombo. To the north, i.e. towards the city center, there are a couple more sandy beaches: Wellawatte Beach and Dehiwala Beach. But we would not advise staying there, because they are dirtier and crowded with boats. If you move even further to the very center, there are still a few places conditionally suitable for swimming, but only local kids swim there, who don't care about the cleanliness of the beach and water.