Drinking too much water can kill, yes, but we're talking a lot of water, not the amount you can just casually drink and not realise that you've had too much.

The relevance of those who are exercising a lot and drinking a lot- the sodium levels can drop to a dangerous level, but the average persons workout does not leave them at risk. It is marathon runners, or those in hot conditions working out hard that kind of level.

More often water toxicity is due to the amount of water taken in after taking ecstacy.

My point is, don't panic that you NEED sports drinks and you'll die if you drink more water just because you're exercising for a bit longer or on a warm day.