I'm not sure whether this is bug or feature, so: I noticed Deep One terrorists are super-deadly in OpenXCOM. They can kill aquanauts with one shot on Experienced difficulty, even if they wear Mag. Ion Armour. When I MC'd one in the final T'leth mission on Veteran, it could kill a lobsterman with two hits. As such, Deep One attack is on par with Sonic Canon, except they have auto-fire capability. What is more, they can also reaction-fire now. It's been a few years I've played original TFTD, but as far as I can remember, armored aqanauts were pretty much immune to Deep Ones.
You can test it yourself, I'm attaching a save from ship terror mission on Experienced. When you end turn, two Deep Ones should emerge in front of aquanaut Dubaku Rajuli and behind his back. Deep One reaction fire is particularly deadly, it usually kills an aquanaut on the first hit. The save is from 2016-06-15 nightly.