How could 11-year-old Harry do that? He didn't even own a wand until later that day, and had no idea how to use it.

I think it's better (more fun) that we leave the Dursleys to manage it for themselves.
Most likely, that muggle who let them use the shack went looking for his boat and found them stranded there.

Like I said two-and-a-half years ago, I thought the point was that we don't know or care.