Shortly after takeoff, the acft's climb performance began to deteriorate and the R eng started to overheat. Subsequently, the R eng was feathered and the crew attempted to return to the arpt. The radio transmitter bcm unintelligible, so the tower psnl cleared the acft to land on rw y 10 and asked the crew to key their mike twice, if they understood. The mike was keyed twice. The acft entered a R dwnwnd for rw y 10 at a very low alt. According to witnesses, the acft went into a steep bank as it was turning onto a base leg over water, the R wing tip hit the water and the acft cartwheeled and crashed. The coplt rprted that after feathering the R eng, the VMC of 84 kts was maintained; however, he believed the acft stalled just prior to impact. A teardown of the R eng revealed the edge of the #8 piston and its piston rings had failed. Metal filings were found in the oil and the oil pump was scored. There was evidence of oil starvation to the crankshaft journal, as well as piston slap associated with the ring failure.