Karlovassi beach is a small beach next to the road from Karlovassi down to the harbour and close to the hotels in Karlovassi. There is a small tavern by the beach and sunbeds/umbrellas are available.
The beach consists of sand and pebbles. Unless you just want to go for a quick swim you're much better of continuing to the beach at Potami.
From Karlovassi follow the road towards the harbour.