I was pleased to read that tidal turbines for power generation are finally supposed to be installed (for test only at the moment) in the Cook Strait this year – though I can't find any info on whether it's actually happening at the moment. I suspect some doubters of the scheme might be confusing tidal power with the much less proven idea of wave power, because tidal power has been working well overseas for a long time – in fact one French plant has been operating since 1966!
Having paid off the cost of construction it now produces power very cheaply – cheaper even than nuclear there, and cheaper than NZ's hydro power.
Estuary dams like that do have an environmental impact on the fish that use the river, which was a factor weighed in the consideration of the finally-approved Kaipara tidal power station project up north, but shouldn't be a problem for the Cook Strait turbines (I suspect the rough conditions might!). All together though I think it'd be less than the impact of the other power generation options (perhaps excepting wind).
The Cook Strait gets an enormous tidal flow back and forth due to the interesting tidal patterns that basically rotate around and around New Zealand (so when we have high tide on one side of the main islands, we have low tide on the other, cycling around) – Wikipedia has an excellent explanation of why we get these two tides a day around New Zealand which also briefly explains why some places in the world only get one.
Tidal power is considered very predictable, although in the more extreme weather conditions the Cook Strait flow can reduce to the point it stalls.
But it's not at all constant – the pattern is affected by many factors (the biggest being the position of the moon and the sun), so tidal power wouldn't quite count as ‘baseload’ generation (available whenever we need it). But if the trial is successful and a full-scale project goes ahead, it should at least conserve the precious baseload hydro water, and hopefully reduce the number of times we need to use our fossil fuel ‘plan b’ stations.