Tidal and Wave Power

Slightly different thoughts today. I was reading about tidal and wave power and it looks like it could be a viable technology that could provide a significant (15%?) amount of power, clean power. But then I got to thinking...

One person talked about how mother nature is providing tidal power for free and we are just wasting it by not using it. But, what did they mean by that? How is tidal power being 'wasted'? In other words, if we aren't using the energy, where is it going?

If a tidal power generation operation were to take 50% of the energy from the tide, that means that 50% of the energy the tide used to provide is not being used naturally. So, if we take tidal energy, what effect does that have on nature?

Lots of questions.

My first thought is heat. Eventually, all energy is lost as heat. Therefore, current tidal energy must be being converted into heat. Is that heat warming the water? Will the removal of the tidal energy result in lower water temperatures thereby affecting life in the water? Tides also cause mechanical erosion so a reduction in tidal energy will slow erosion - that's probably a good thing but I'm not sure if erosion provides benefits to sealife in the form of releasing minerals?

I think back to when people would have looked at an untouched forest of trees and marvel at all the wood they could cut down and then eventually cut the whole forest. Or when the nearly inexhaustible supply of cod on the Grand Banks off Newfoundland let to the virtual destruction of it. By siphoning tidal power on a large scale, what effect will we have?

Opinions and links to resources would be appreciated!