Although I do not have any personal experience with this type of cable, unfortunately one thing I can say is that you need to be in the business to identify them in detail by eyeball from a pic - and even more so from what amounts to remains.
The problem is that undersea power or data cables look (to the untrained eye) very similar, because the majority of the structure is actually armour and reinforcement around the active portion.
Even a single fibre-optic cable for undersea use is as thick as your arm due to the armour and waterproofing. Even the 'lightest' have to be able to offer at least token resistance to being dredged up by errant anchors, and other mistakes made above them.
I thought I had made a lucky strike earlier, with a defence document that claimed the information it contained (which included drawing showing the cables' locations) dated back to the late 1980s
- it's hard to date those little buildings of course, but they are reasonably fresh and clean looking, so 1980s would be reasonable as a date they were in use.
But no luck.
Although numerous cables are shown, and some for the islands in the area, there is nothing at the location being considered:http://www.scotland.gov.uk/Resource/Doc/345830/0115110.pdf
(You might want to download this, as I suspect it will disappear at some point.)
From this we can probably conclude that this site predates the 1980s, and that it was out of action by the 1980s, or it would have been detailed in this doc - not much, but at least it might be something to help focus more detailed searches on earlier dates, and have some confidence.
It's not likely to be later, as the doc also states it includes cables in place up to 2004.
We should not forget the possibility that the there could have been an MoD or similar facility on the site, possibly unmanned, and the two buildings were merely substations providing terminations for underground power cables, and that there is no undersea aspect to this site.
If it was Cold War era, it is unlikely to appear on any OS mapping, since the MoD has only abandoned the old policy of not marking its installation on such mapping recently, say the past decade or two in round figures, so if this site was abandoned as the Cold War fizzled out... it will not show up on any later OS maps.
At least, not with any description, although the presence of the small buildings may be recorded at some future time.
I'm not suggesting ruling out the undersea aspect - merely suggesting a little thinking 'out of the box' and some more open minded thoughts, just in case