Unawatuna vs Mirissa — which should I base myself at for the south coast beach portion of my trip?

Both are on the south coast and within 40 minutes of each other — you can absolutely day-trip between them. But they have distinct characters:

Unawatuna:
More established, more infrastructure, more restaurants and cafes within a small area. The beach is a protected bay with calm water and moderate snorkelling. Has been heavily developed — large resorts now crowd the beachfront and the "cute village" character of 10 years ago is largely gone. Still perfectly nice but overdeveloped for what it is. The snorkelling at the Japanese Garden (a 15-minute swim from the beach) is genuinely good.

Mirissa:
Smaller, younger vibe, the beach is a perfect crescent with a headland at each end. The whale watching industry has brought money but it's less built-up than Unawatuna. Better sunset view. The eastern end (past the coconut tree headland) is quieter. Food options are slightly less varied than Unawatuna.

Hiriketiya:
This is where I would stay. A horseshoe-shaped bay 20 minutes from Matara by tuk-tuk. Excellent body surfing waves, a fantastic daily food scene (the cafes lining the bay are seriously good), and the crowd is predominantly surfers and long-stay travellers rather than package tourists. It's what Unawatuna was 15 years ago.

For your priorities (snorkelling, walking, sunsets):
- Snorkelling: Unawatuna (Japanese Garden) or Pigeon Island if you make it to Trinco
- Long beach walks: Mirissa or Hiriketiya
- Sunsets: Mirissa (faces west)
- Quieter: Hiriketiya