"Sea Within a Sea" has been viewed as the Horrors' shift from the garage rock of their 2007 debut album, Strange House. Jacob Sheppard of DIY magazine examined the song as the group's "new direction", where they "take on a more mature sound; more electronic, bass heavy and psychedelic", hailing it as a "Neu! sounding, shoe-gazing seven-minute masterpiece". Similarly, Cam Lindsay from Exclaim! magazine has regarded the track as "an eight-minute beauty that discards the raucous trashed garage of old" and "sinister throwback sound in deep krautrock trances and '60s psychedelia whilst still managing to hold on to their spooky shtick".

Many music writers and critics have stated that the song combines influences from krautrock, post-punk and Portishead (the previous band of the single's producer, Geoff Barrow). Johnathan Garrett of Pitchfork described the song as an "oddly muzaked take on krautrock"; however, he commented that the track was "weirdly restrained" and "the Horrors can't even be bothered to leave an impression". Sean O'Neal of the A.V. Club depicted that the song's instrumentation "drowns its post-punk ebbs in sprays of crashing guitar before being carried out on a sparkling tide of minimalist electronic pulses". Music writer Dorian Lynskey of the Guardian claimed the single as "ascendant krautrock - Can by way of the last Portishead album", praising it as "several kinds of wonderful".Prefix magazine explained that the song "perfectly connects with Portishead touchstones such as Neu!, Can and Kraftwerk" and is a "glacial eight-minute groove that only gets better as its parts disentangle". Emily Tartanella from PopMatters expressed that the lead single was "miles away from anything they’ve done before", outlining that "like Ian Curtis fronting the Cure, it’s mysterious, languid, and just a little bit spooky."NME magazine described the song as "an eight-minute Spacemen 3-meets-Neu! odyssey of ominous motorik rhythms, Faris' mournful incantations and an expanding starfield of synths". In October 2011, NME placed it at number 101 on its list "150 Best Tracks of the Past 15 Years".

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