"Bear in mind that the functional generating equipment has a design lifespan of around 30 years therefore in the lagoon's predicted lifespan this kit would need to be changed out in it's entirety multiple times, accruing further major operating costs."
Without a dramatic improvement in materials science maintenance will be a huge cost. Ship propellers suffer from the hostile chemical environment of the sea--do planners think that these turbines will be made of some magical material that can do better?
It might also be wise to scrutinize the close cooperation of industry and government officials in cases like this. Bribes? Invest in a trustworthy independent study, perhaps from a different country, before proceeding.