The impostor swims to the beach and walks out of the water soaking wet, clothes dark, dripping water. He walks a few feet and starts to kneel. In the very next shot, a continuation shot of him kneeling, he and his clothes are completely dry.
When the impostor is dragging the real Mr Howell (Jim Backus) away (after knocking him unconscious), a Jim Backus plastic face mask is easily seen on the impostor.
When the impostor is rescued at sea, nobody recognizes him.
Mr Howell tries to follow the impostor as he swims away; he wants to retrieve his wallet, which is in the pants the impostor is wearing. But later in the series, Mr Howell has his wallet. However, since he is one of the richest men in the world, he probably brought several wallets with him.
When setting off on the pontoon raft, Mr Howell is handed a loaded flare gun. However, the castaways had exhausted their supply of flares in Voodoo Something to Me (1964).
Since the island is not on any nautical chart, it is a major question as to how the fake Mr. Howell knew where it was.