The reason we haven't mapped the seafloor is because why would we? It's like arguing we know nothing about biology because we've only sequenced the genome of a fraction of humans or something. It's not that we can't do it, the reason we haven't done it is because there's no good compelling reason to do it. What do we expect to learn from mapping 100% of the sea floor?
As for the parents question – "How many years until we've discovered "everything"?"
I think we may be fairly close to knowing everything we can know and it's quite reasonable to assume we're now comfortably on the tail end of the S-curve of physics discoveries. I hope I'm wrong of course.