If you're looking for a 2 player game this really won't cut the mustard; I mean, there's nothing stopping someone from playing it with 2, I just don't see if being much fun (kinda like 2 player Power Grid). Heck, this game isn't very good with 3 either. 4 or 5 is really the sweet spot.
This game shines best in a chaos-filled river with lots of boats in play. With more people in the game there are more gems to try to steal from others, more cards to keep the potential speed of the river current at a reasonable pace. (The way the river moves is the lowest number tile selected by all players (after possibly being modified slightly by the weather) will be used as the current speed for each round) With my band of gamer misfits even with 5 of us we'll all select our '6' tile occasionally causing the game to drag as we all spend our gems recovering boats that went over the falls. With two people that river will fly and the game will likely drag on for far longer than you'll want to play.
The other problem has to do with the winning conditions. You win if you get 4 gems of any one color, one of each 5 colors, or 7 gems. The 4 of one color will happen pretty easily with 2 people. You'd likely have to reduce the amount of gems you start with as well.
It's a fun, light, easy to teach game though. If you have 4 or 5 people its great for a quick game.
(but german) variant for 2 on 
