I had a hydrodynamicist friend working on tidal and wave energy. Basically, it's hard. Everything gets rusty, barnacled and breaks in storms.
Funding... If you throw money at hard problems you have a chance of solving them. If you don't, you don't. Nobody will be making money from tidal schemes for a little while.
Then you need to get power back to the shore which is very expensive... It's interesting to see that offshore wind-farms are only just starting out, even though much of the technology was proven on land long ago. A big problem is simply getting maintenance people on and off the turbines.