I disagree with much of the above advice.
First, and foremost, avoid rocks and the edge of the river. That is the most important thing. On many screens, you can set yourself up in the middle of the stream and just drift.
On the first go-round, if you float without accelerating, the animals will run out before you get to the end. At that point, speed up to avoid wasting gas, because it always takes more gas to drift the same distance than to speed down the same distance. To test this theory, I sped through the entire game once, and found that, even though I hit several rocks along the way, my gas level at the end was well over 200. Thus, for example, it is ALWAYS best to speed through the dock stages, as long as (i) you don't speed up until you see where the gas can will be; and (ii) you don't speed faster than you can steer to avoid rocks. With some practice, you can jam through at top speed and strike no rocks by just tapping the right arrow a couple of well-timed places.
In second and third rounds, speeding will cause you to miss animals, because the animals last longer than it takes to drift across. To get them all, you must idle or go in reverse for a while.
This business about oks being more points than flawless or good is just dead wrong. If you are unable to move the mouse to get them all, it is better to get a bunch of oks than to get a flawless and a bunch of misses. However, for each kind of animal it is always better to get flawless than good, and better to get good than ok. A flawless lion photo is alway worth more than a good or ok photo. The value does vary greatly from beast to beast, so much so that, for example, an "ok" lion is worth more than a "flawless" toucan. Make sure you get the big ones: lions, rhinos, hippos, hyenas, etc. The small birds and coiled up snakes are worth a lot less.
