It isn't a good idea to swim close to the shorelines. Crocodiles do inhabit the area and there are some huge ones around that wouldn't think twice about making a meal of you. I was told that the middle of the lake was safe to swim in, but you definitely don't want to be too close to a 16 foot man-eater.
There are a lot of hippos in the waters of Lake Kariba. In case you didn't know, hippos are the deadliest animal in Africa (after the mosquito) and are not to be messed with. If you see a group of hippos, it is in your interest to stay away. If you happen to suddenly be in the midst of them, you will know it because they will become extremely agitated and either give you a firm warning or attack. If you get a warning, get away, and don't waste time. They can crush your boat like it was nothing, and in the process could crush you too.
If you are fishing on the lake and catch a pink/orangish hued catfish that "squeaks" when it's above water, be careful of it. On it's fins there are spikes that supposedly can pierce through anything. If you get pricked by one, it will be irritated for months - almost like having an itch you can't scratch below the surface of your skin.
Since Lake Kariba is man-made, there are a huge amount of trees that were not cut down before the lake formed. If you are riding around in a power boat, you definitely need to be careful for them, otherwise you might find your boat getting damaged. The owner of the boat I was on had me watch over the front of the boat to warn him if I saw any. Even then there were some I could not see that we ended up hitting.