Well, I liked the final episode more than I expected. Then I looked at Wikipedia. RTD is credited with writing the first and last episodes, Pete McTighe the three middle episodes. I'd really like to know if Davies rewrote McTighe to any significant extent, because the middle episodes are the weakest, as far as I'm concerned. I'm not sure they were entirely on the same page because one episode seemed to suggest that the aquatic species had been watching humanity for some time, but this one went back to the usual Sea Devils/Silurians routine about them having recently awakened.
Still a mess overall, mind.
Yeah even though I think it's been ok, and at times very good (just not often enough) it definitely feels somewhat incoherent. You're absolutely right about the "We've been watching you for ages"/"We've just woke up" disconnect.
As I've said I do wonder if this was originally conceived as an original story about an aquatic species finally having enough of mankind's shit and rising up and RTD had a lightbulb moment and thought "Hang about, Sea Devils!"
I also wonder if Disney asked if RTD had any other idea and he dusted this off from a drawer.