This episode had everything necessary to be great, but somehow did not click with me. The Sea Devils looked excellent, but beyond that it was all a bit of a load of wet fish. The Sea Devils definitely seemed both way too powerful with their super crystal wiping out the Earth, yet also literal push-overs in a sword fight. I'm not sure if I got to like any of the guest cast.
This Doctor needs to stop allowing innocents to rush in at the last minute to sacrifice their lives to save the Doctor. And then this Doctor keeps on saying "well, okay, if you want to die instead of me then that sounds good" and skips out.
Also not sure how I feel about Dan cutting down Sea Devils with a sword. Knocking a Sea Devil overboard with a block and tackle or something seems fine for a companion, but when a "nobody" who used to hit Sontarans with a frying pan starts slaughtering bad guys two at a time it doesn't seem so heroic. You're not supposed to join the Doctor to learn how to be the ultimate killing machine.
I had high hopes for this one, but the tone and the characters just did not do it for me.