During the 1864 election campaign there was a dispute about whether McClellan was on the armoured corvette USS Galena during the last three days of the Seven Days Battles (30th June, 1st and 2nd July). Of course neither the extreme Democrats (who tried to argue he wasn't on the Galena) nor extreme Republicans (who tried to argue he spent the whole period on the Galena and contributed nothing to the campaign) were right.
Now of course it's accepted he spent a lot of time onboard USS Galena. However the notion that he did not influence the battle from there is questionable. Stephen Sears denie McClellan had any telegraphic communications with his army and believes McClellan had "lost the courage to command" (Gates of Richmond pgs 280-1). Of course he is utterly wrong in this as Lt Clum's signals detachment accompanied McClellan and kept him in contact with his army. One must speculate how he missed a lot of evidence that completely undermines his argument.
The evidence of the assistant surgeon of USS Galena, himself apparently quite an extreme radical Republican gives some detail about the signals. See http://books.google.be/books?id=-_EsAAAAYAAJ&pg=PA125
One of the most important pieces in this testimony is what the message that brought McClellan ashore. There were two messages close together reading:
"McCall is breaking"
"Sumner is having a hard time."
These messages (apparently from Porter) caused McClellan it immediately return to shore and make a beeline for Sumner. By the time he arrived Sumner and Heintzelmann had moved forces and had repelled Longstreet.
The timings of McClellan's movements are confused in the literature. Even Ethan Rafuse (considered by some to be a "McClellan supporter", something I don't really agree with) suggests McClellan didn't arrive onshore until 9-10pm, which given Heintzelmann's 8pm meeting with McClellan seems to be much later than he came ashore.
On thing that is notable is that the next time McClellan had to go to a meeting with Rodgers he sent a very specific note to Sumner that he was in command due to seniority. Why? I would hazard he was unhappy that Sumner didn't "step up" as he automatically should (see Sumner's testimony to the JCCW: http://archive.org/stream/reportjointcomm02wargoog#page/n363/mode/2up ) and spelt out that as senior Corps Commander he was also 2i/c, although responsibility seems to have been something Sumner made a virtue out of avoiding (just as Franklin and Burnside made a virtue out of ignoring orders).
More work needs doing on McClellan's movements that day, and what signals he sent and received from the Galena. I suspect a very different picture that Sears et al. will prove to be correct.