Kreacher! Kracher helped him, that's why he's so off. Maybe Regulus did a memory charm on him so he doesn't remember, but Kreacher would have done anything for Regulus, he was the "good son". Even drinking evil potion.
That's an interesting theory! I try not to think about Kreacher more than I can help, though. The interesting thing about your idea is that Kreacher might have been able to use the boat as well without "registering" as a wizard, as Harry did. I have heard some people speculate that Kreacher hoarded the locket, too.
I was half expecting Dobby to die in the most recent book. Glad he's made it okay so far.