This happened last summer. My parents had rented a big house on the beach so we could spend two weeks in the sun. I had invited my cousin Sam who is 16, like me. My brother, Sam and I slept in the same room. On our first night we couldn't sleep so my brother, Sean, suggested we go to the beach. We got up and took some towels. It was empty: not a single person was there. My brother said that he wanted to swim naked and I immediately said yes. We took our shorts off. I was half rigid. My cousin was watching and was surprised. I could see he was having a hardon. I said that it was ok and that nobody would see us. He finally accepted after 10 minutes and he took his shorts and boxers off. Damm what a boner!!! He has a 7 inch cut penis and my brother and I are also 7 and 7.5 inches. I was also fully erect by now and my brother proposed we masturbate. We immediately started wanking our cocks. I came first in 5 minutes and my brother and Sam were jerking each other by now. I then go in the water and they followed me. It was one of our best experiences although my brother and I masturbate together all the time at home.