Santorini is not necessarily an island famous for its beaches like Naxos or Rhodos and the likes. People come here for the spectacular views and the whitewashed villages along the steep cliffs. In fact, there are not many places on the island where you can get close to the ocean. The little harbor at the bottom of the cliffs is one of them but the others are a good 45 minutes drive away. So is Kamari, a town that consist mostly of vacation rentals and a few restaurants along the shore. The beach is made of pebbles and is not particularly pretty but hey, you are close to the ocean if that's what you want.
You can get a public bus to Kamari from the central bus station in Fira.