How much hot water does a nuclear plant put into a river or ocean each year?

Quite a bit, but you would need to look at specific plants to get an accurate reading.

More worrying is the discharge of Heavy Water into the river. This is permitted by law aparrently.

If ingested, it is absorbed and used by the body as normal water is, but causes cancers and abnormal cell behaivoir.

What goes around - comes around.

It does occour naturally. Germany was developing heavy water plants in the 40's, and it may be a fuel of the future if everything else runs out.

Hot is a bit relative. The water is colled prior to discharge even when piped into the ocean.

Very little since most of the heat is rejected by means of cooling towers to the atmosphere.

I don't know... but technically I believe it just heats the water... doesn't add any water.