Tomb Raider III
In Tomb Raider III Piranhas can be found in India. Some of them will let Lara cross a river, but in the level Crash Site they are more aggressive and will stop you from doing so. However you can create a diversion to get through anyway.
Naturally they can only be found in the water. They can not be killed. You just have to outrun them.
Shadow of the Tomb Raider
The only way to avoid them is by hiding in thick patches of underwater plants. If they attack Lara, the only way to get rid of them is by quickly climbing out of the water. If Lara can't get out quickly enough they will bite her and kill her within seconds.