I emailed José Antonio Pérez in Spain, congratulating him on the lego turbine he made and could drive with a hair drier. Hi José!
Meanwhile a turbine was installed in Northern Ireland to take advantage of a huge tidal flow ..
It's a classic design, that required a floating crane to be borrowed from Norway to install. The company website is http://www.seageneration.co.uk/ but search in Google for 'SeaGen Strangford Lough' for other articles starting in 2007 (it took a lot of planning to install).
Using the VAWT design, you could have a much lighter setup. It could float for one, and be anchored upstream using two tethered lines..
Illustrated are three VAWTs. A Buoy keeps it afloat, with a counter weight at the bottom keep it the right way up. There are two cables to anchor it upstream. They might have to be stays to prevent it fowling the blades, because at each change of tide, the contraption is going to relocate to the a new position slowly. This does not need a crane from Norway to install. Indeed it may well be easier to clean the inevitable lifeforms that attach themselves overtime, by floating it to the surface (still tethered, but now flat to the surface).
Whether a river or a tidal flow, the current is going to be strongest in the center of the channel. If not in a channel, then it doesn't really matter about the position of the blades to take advantage of greatest flow.
Unlike the the same VAWT design for wind farm, the device needs no constant orientation to the flow - underwater the tide flows in only two directions, and the VAWT will tune to that automatically.
Please forgive the crude graphical representation of a river bed / water etc.