I race sailboats. Getting involved in an active fleet that hangs out afterwards is the single best way I have found to make friends. At one point in NYC I was racing on a J/24 part of a crew of 5, we raced against 15-20 other J/24s and all met at the docks after the race, every Tuesday from May until October. You are bound to make friends. Eventually you start getting invited onto other boats.