While the Doctor later happily treated Sarah-Jane Smith to a rendition of Oh, I Do Like To Be Beside The Seaside in anticipation of a nice relaxing holiday on Florana, it's doubtful he was as delighted to be beside the Sea Devils earlier in his personal time-line!
Back then, on a visit to see his old nemesis the Master (who was by then being held in a maximum security island prison as its only inmate having been captured by UNIT at the end of The Daemons- his efforts to harness the power of Azal thwarted, had turned into a race to stop him breaking out), the Doctor is surprised to find that the time to reflect on his actions has seemingly changed him- perhaps he secretly enjoyed the quiet life of a country vicar?
MASTER: He's quite a decent sort, really. Used to be the Governor of a colony once, so he tells me.But after his visitors leave it's revealed that he's in league with the very man supposed to be supervising his imprisonment!
DOCTOR: Very small colony, wasn't it? In fact, I believe they claimed independence soon after he arrived.
JO: He seems to be looking after you very well.
MASTER: Oh, yes, I've got everything I want, except, of course, my freedom.
DOCTOR: You can consider yourself lucky. Quite a few people were in favour of having you executed.
MASTER: My dear Doctor, don't think I'm not grateful. I've had a chance to think about things while I've been in here.
DOCTOR: Have you now?
MASTER: I wish that something like this had happened a long time ago.
JO: Surely you don't like being locked up?
MASTER: No, but, it's given me a chance to reconsider my life.
DOCTOR: Am I to take it that you're a changed man?
MASTER: Is that so very incredible? After all, I do have a great deal of which to repent.
TRENCHARD: There we are, then. Fooled them nicely, didn't we?He'll get the television he requests later, so he can watch The Clangers. As you do.
MASTER: I hope so.
TRENCHARD: What about that hypnotism wheeze, eh? That took them in.
MASTER: Don't underestimate the Doctor. Do you really believe that he came here to see me?
TRENCHARD: Why else?
MASTER: Vanishing ships, of course.
TRENCHARD: Oh, I don't think so. No, he didn't seem particularly interested when I mentioned it.
MASTER: You did what!
TRENCHARD: We were just chatting. No harm done, surely.
MASTER: Let's hope not. By the way, what about those admiralty charts I asked for?
TRENCHARD: They'll be here this afternoon.
MASTER: Ah, splendid. Er, there is one more thing.
MASTER: I wonder, do you think I could have another television set? For the bedroom.
TRENCHARD: Yes. Yes, I'm sure that's possible.
MASTER: Colour, of course.
TRENCHARD: Of course! Well, if you'll excuse me?
The memory of that happy discovery of a rather interesting extra-terrestrial life-form quite possibly spurs a later incarnation to marvel at the Teletubbies! But there's the small matter of strange sea-monsters in fishnets attacking Navy ships to consider, too.
Bet you can't guess who's controlling them?
Ultimately, though, the battle to harness/stop them ends tragically for the Sea Devil species as the Navy's finest take them out.
At least the Doctor had tried the diplomatic route.....
SEA DEVIL: This is our planet. My people ruled the Earth when man was only an ape.Of course they will return to provide a blast from the past for the Fifth Doctor in Warriors Of The Deep alongside their aquatic cousins the Silurians as part of Season 20 of Doctor Who, a celebration of twenty years of TARDIS travels. Looks like it was a hell of a battle, too!
DOCTOR: Your people went into hibernation and abandoned Earth to its fate.
SEA DEVIL: Our astronomers predicted that a great catastrophe would end all life on the face of the Earth.
DOCTOR: Yes, but the catastrophe that you predicted never happened. And the apes that you left behind on the surface to die became man.
SEA DEVIL: You know our history?
DOCTOR: Yes. Yes, I've encountered your people before. That is why I want to prevent a conflict that can only end in your destruction.
SEA DEVIL: We shall destroy man and reclaim the planet. Already we have begun to sink his ships.
DOCTOR: Yes, and already more ships are being sent to hunt you down.
SEA DEVIL: The submarine? We have captured it.
DOCTOR: You may win a few victories to begin with but eventually you're bound to lose.
SEA DEVIL: There are many thousands of our people in hibernation in this base. We have other colonies hidden all round the world. We shall be the victors in the war against mankind.
DOCTOR: But there's no need for a war. Why can't you share the planet?
SEA DEVIL: That would be impossible.
DOCTOR: The depths of the sea and those areas on Earth where man cannot live can be yours.
SEA DEVIL: And man would agree to that?
DOCTOR: There's a chance. Wouldn't it be better to try for a peace, than to launch yourself into a war that you cannot possibly win?
SEA DEVIL: I will consider what you have said.
DOCTOR: Let me return to the humans, and I will endeavour to make a peace for you.
SEA DEVIL: Perhaps it would be possible.
They are now seemingly consigned never to return in a New-Who context- unless Series Nine is deemed the right moment for them to rise once more, as the Silurians did from The Hungry Earth/Cold Blood.