My answer is a bit off topic to your actual question, but addresses something you said:
oceanographers have stated that less than 5% of the worlds' oceans has been explored
I would venture that less than 5% of the ocean floor or deeper ocean depths is perhaps where less than 5% has been explored.
We've really been over the vast majority of the ocean surface at some point (and have tools like satellites that give us significant data on the rest). Here's the current location of floating buoys:
It looks pretty comprehensive near the surface! Still, there's often hundreds of miles between sites, so while it means we have\do cover a fair bit of the area (more than 5%!), we aren't everywhere at once.
And it's important to note that a a significant part of thermohaline circulations are near the surface, generally the red areas on this map:
But the big thing to take home is that even where we don't have as much data, we can see the large thermohaline circulations well because of its scale and relative continuity. It's quite comparable to the upper atmosphere, where we also don't get data on large parts of the upper atmosphere, but can rather well diagnose jet stream fluctuations. Here's the upper air sounding network:
(Weatherstem.com upper air observations lesson)
Both networks are aided some by satellite and ship/plane observations, but overall, despite fairly limited observations, the circulations are quite evident because of their size and impacts. So we have a pretty good handle on what's going on.
Hopefully someone else is able to better address your central question about why it is vital to life on Earth!