The price is about 700 drachmas for 1bed and the same for 1 umbrella. I have visited every beach on Poros and will give you a description of them all. Click on the names on the map and you will get there. After every beach you can always come back to the map if you like. They are presented from left to right as they are on the map.
Some people prefer to swim in a pool.
Use the links under the map. The best ways to reach the beaches is by motorbike.