He was spotted there, as it turns out. Reports have him backing into a part of the launch ramp and conversing with marina employees. He couldn't very well deny being there.
all the "evidence" that has come out is IMO purely circumstantial.
That's the problem for SP, there is a mountain of circumstantial evidence pointing to him. Possible eyewitnesses at the Berkeley Marina that may be able to testify to contents inside the boat.