I Will Ask You Questions - Now with MORE fantasy ecology!

Can I have another?

Sure.
Imagine that we were horribly wrong about the position of the mantle, and there is actually an 'undersea' below the ocean floor, with various cavernous openings into the seas. This undersea is occupied by a race of sentient cephalopods who are just as ignorant of us as we are of them, but have progressed to a technological level that is, on average, around fifty years behind ours. What would their first impressions of us be, and what barriers would exist between our interactions with them, aside from distance, pressure, and other environmental constraints?