Boossa Beach is about a kilometre wide. It's a little bay tucked in-between Hikkaduwa and Galle. Getting there is pretty easy if you take the highway.
The Western stretch of the beach is mostly rocky, but the Eastern stretch is a beautiful sandbank that can be mostly undisturbed, by the standards of beaches in the South Coast near Galle. We went there a little off-season, so granted, it was less crowded than what must be its usual. Locals were out and about in the sand, kids were playing football and friendly stray dogs came over to be petted.
This stretch is largely devoid of big tourist establishments, except for one big boutique hotel at its Eastern corner. Most of the oceanside real estate is controlled by small hostel/cafe type establishments that can offer extremely cheap rates if you look in the right places. I'd suggest booking.com or a similar site.
Be warned though, this being the South Coast, you can expect to encounter plenty of racism if you're a local. So maybe you want to make sure the owners know you're coming and are OK with that. Off season, when tourist flows dry up, greed trumps racism, and chances of the brown skinned getting a room go significantly higher.
We walked along the beach at sunset and found plenty of driftwood, there was also this pitch black variety of sand covering the surface in some areas, it produced amazing contrasty colors with the yellow sand underneath it when we mucked about on it.
The beach is scattered with fishing boats, waiting for their owners to come and release them into the ocean when the time is right. With the monsoon in full swing, the seas were rough, and no one rents surfboards from the hostels right now, but I guess you can find decent surfing on this stretch during the season.