Ok, for once I'll quote the author since he summed the game up so nicely!
In Upstream, you control a little diver and have to retrieve lost treasure chests from the bottom of the sea.
The trick is that you can't lift the chests - you can only push them horizontally. To reach the boats on the surface, you have to use the upstreams in the sea (the little up arrows). When there is no upstream go high enough, you can fill the gaps with air bubbles - they lift the chest one tile and disappear, so you have to think strategically using them.
Thank you, the author! Now go play his game!