You see a boat filled with people. It has not sunk, but when you look again you don't see a single person on the boat. Why?
I think the answer is
You don't see a single person (unmarried), instead all of the people on the boat are in a married couple.
I've actually been on a boat where this has happened:
The people on the boat are all scuba-divers and they've all gone into the water and are now underwater.
The answer could be:
The boat is docked and everybody had disembarked.
The boat is a submarine. It is full of people, but they're all in the boat, not on the boat. (It could work with a regular boat as well, provided everyone is below decks.)
The answer is
everybody jumped off the boat
the boat was on fire.
they all decided to go swimming.
A multitude of persons on the boat.
That is because
the boat has capsized!
This is a Schrödinger Boat. Depending on when you look at it and when its quantum superposition ends, the persons may or may not be here.
The people are playing a pretty epic mass-multiplayer game of hide and seek
You were hallucinating when you saw the people
You were hallucinating when you thought the people had gone
You were hallucinating both times. There is no boat!
Another simple solution:
You returned to that place after a long time. The boat is still there, the people have left.
The boat is in waves that rock the boat towards and away from you. So one minute, you can see the decks full of people, the next you just see the hull.
"when you look again you don't see a single person on the boat". Of course you wouldn't see 'a single person' because the boat is full of people. Too much thought when the words are right there!
A fog bank has rolled in, obscuring your view of the boat's deck
It is clearly the case that:
the sails were just furled. Now, there is nothing blocking the Sun from shining right in your eyes and preventing you from looking clearly in the direction of the boat.
This one is a bit dark:
The boat is filled with people, but they are not recognizable as single persons because they are all blended in a huge pile of human jam.