1. The ocean is something like 140,000,000 square miles, Jimmy. I know math is hard...but can you even begin to comprehend how much water it would take to make that 140 million miles^2 6.5 inches deeper? And considering how high water's specific heat is...and how much energy it takes to phase change it to liquid to begin with...the amount of energy we're talking about is simply mind-boggling. Yes. Horrifying. Stop and think about it for a minute.
2. In order for man to have had no effect...there would need to be some sort of intervention from a supernatural actor. It's not opinion, it's not supposition. The belief that humanity adding millions of tons of greenhouse gas to the atmosphere, massively dropping the pH of the oceans (carbonic acid. Look it up. It's what happens when all that carbon gets soaked up by the oceanic carbon sink), and more or less pumping mind boggling amounts of extra energy into the system...has no effect? Because....magic? I guess? Madness.
It's like believing dropping a Tide pod into your fish tank would have no effect.