In this case, necessity is clearly the mother of invention. With another planned script having fallen through on short notice, Terry Nation was asked to write a six-parter on incredibly short notice. Keep that in mind when you watch not just "The Sea of Death", but all of the subsequent episodes, because it's clear that the true skill on display here is one of economy. As soon as the Doctor and his companions (and at this point, it's pretty clear that the antagonism between them is mostly in the past--they're friends and fellow travelers, not a kidnapper and his victims) leave the TARDIS, we get a series of scenes that establish the narrative with great speed. The alien world and its acid seas get a quick scene of set up, the villains are shown with a minimum of dialogue and screen time to be ruthless and murderous and yes, actually a little bit creepy in the way that silent people wearing black vinyl outfits and holding knives are generally creepy. Everyone generally takes this opportunity to point out that they're not as scary as the Daleks, but I kind of feel like that's a bit unfair. Judged on their own merits, they're pretty effective.
The central conflict is set up with equal economy...or at least with as much economy as something so blatantly contrived can be. "We have a machine on our planet that makes it impossible to be evil...and before you ask, the bad guys are immune to it...and before you ask, we can't just wreck it because we're hoping that we can fix it...and before you ask, we can't just keep working on it because we can't let it fall into enemy hands...and before you ask, we have to fix it now because we've found a way to make it work on the bad guys...and before you ask, we can't just make new keys because they've got super-special codes that would take thousands of years to replicate." To his eternal credit, George Coulouris makes the scene work by playing up the ludicrousness of the concept just a little bit; he suggests through his performance that Arbitan may not be the most trustworthy of Wise Mentor characters. It's as though he's daring everyone else in the scene to point out what a crazy idea the Conscience is in the first place.
And then we cut directly from his impassioned plea for help...to the main cast walking back to the TARDIS, feeling just a titch guilty about having turned him down flat. It's a scene that is utterly drop-dead hilarious to modern Doctor Who fans; the thought that the Doctor would respond to a planet under threat from evil dictators with, "Eh, sorry, that's not really my bag," contravenes not just the character but the basic laws of drama. It's no wonder that Arbitan railroads them into the story... again, with amazing economy. Invisible force field around the TARDIS, teleporters that pop the cast from plot point to plot point...it's contrived, and it feels contrived, but you can't help but be impressed by the sheer streamlined efficiency of it all.
It's definitely a case of everyone dancing as fast as they can, and it shows in the production design (there's at least one, if not two, laugh-out-loud special effects sequences). But it's also an episode that hits all the beats it needs to hit without lingering around, and it's about to do in six episodes what took Tom Baker a full season. Not too shabby for a rush job.