Getting to the south coast is one question. Getting around it once you are there is a different one. The main towns between Galle and Tangalle are all connected by the same coastal road, which makes transport simple in theory. In practice, the right option depends on how far you are going, how much you are carrying, and whether you want to move on your own schedule or someone else's.
Tuk-tuks for short distances. For anything under 20 minutes, a tuk-tuk is the default. They are available everywhere, they are cheap, and they do not require planning. Always agree on the price before you get in. Within a town such as Weligama or Mirissa, short rides cost 200 to 400 rupees. Between nearby towns, for example Weligama to Ahangama or Mirissa to Hiriketiya, expect 500 to 900 rupees. For longer stretches like Galle to Mirissa or Weligama to Tangalle, tuk-tuks are technically possible but inefficient. Use a bus or car for those.
Buses for longer stretches. The southbound bus from Galle runs along the coast road through Unawatuna, Ahangama, Weligama, Mirissa, Matara, Dickwella, and toward Tangalle. You flag it from the road, tell the conductor where you are going, pay on board, and get off at your stop. The fares between most south coast towns are between 50 and 200 rupees depending on distance. Buses run frequently during the day, less so after around 8pm. They are crowded during rush hours. They are perfectly manageable for a solo traveller with a daypack. They are less practical with large luggage.
Scooter rental for independent exploration. Renting a scooter gives you the most flexibility of any transport option. You can stop wherever you want, go at your own pace, and cover several towns in a day without depending on anyone else's schedule. Rental costs range from roughly $6 to $10 USD per day in Weligama, with discounts for longer periods. Before you rent, there is a legal requirement worth knowing about clearly: to ride anything over 50cc in Sri Lanka, which includes most rental scooters, you must hold a motorcycle licence from your home country. You also legally need a valid International Driving Permit with motorcycle endorsement and a local temporary riding permit arranged through the Automobile Association of Ceylon. Many tourists rent scooters without all of this in place. That is a real risk on Sri Lankan roads, which are unpredictable in ways that less crowded roads are not. Decide accordingly and with full information.
Private driver for longer journeys or multiple stops. For a full-day trip that covers several places, such as Galle, Ahangama, Weligama, and Mirissa in one go, hiring a private driver for the day is a practical option. Most guesthouses can arrange this. Typical rates for a full day with a car and driver are around 8,000 to 15,000 rupees depending on the distance covered. Drivers can wait while you spend time at each stop, which removes all the tuk-tuk coordination. For families, couples with luggage, or anyone covering a lot of ground in one day, it is usually worth the cost.
The train for the scenic option. The coastal train runs from Colombo through Galle, Ahangama, Weligama, and continues east. If you are making a single longer journey and are not in a rush, the train is the most enjoyable way to do it. Second class tickets cost around 100 to 200 rupees for short hops between south coast stations. The train does not stop at every town and the schedule means it does not always fit around your plans, but for a morning journey with a sea view and no bags to manage, it is worth it at least once.
One combination that works well for most travellers is using buses for the longer hauls between towns, tuk-tuks for everything local, and arranging a private driver for any day when you want to cover a lot of ground or have early morning commitments like a safari or whale watching departure.