When you say "huge amounts", how huge? Like "x times the cost of Hinkley Point C"? I'm only aware of fairly cheap failures so far, but I admit I haven't really tried to follow the subject.
Joking aside, tidal has been a subsidy money pit that has attracted chancers and grifters for a long time. That’s not to say there weren’t honest projects (the link I posted being a good example) but nuclear is a proven, well understood tech that doesn’t need to be developed. We know how to do it.
Hinkley C is far too expensive for many reasons but it will generate steady power for many decades once built. Tidal is basically a non-starter. A tech that has been not been commercialised, that offers only low density power generation and which will inevitably be maintenance heavy. It’s a non starter.