Hydrobot

From Terminator Wiki

Connor and a Hydrobot

The Hydrobot attacks

Hydrobots are Non-Humanoid Hunter Killer units produced by Skynet sometime before 2018. They have a worm-like structure with a claw at their head. They are used to patrol the waters. Within the Terminator universe, they are based on the present day model of Skynet Research's Aquatic WorkBot, which was used to maintain kelp and seaweed, find minerals and oil, and seek out organic matter. ^[1]

As their name implies a Hydrobot has difficulty operating on land, but they could jump incredible distances and use their metal teeth to cut through even metal ^[2].

Behind the Scenes

HKs similar to Hydrobots are in concept art for Terminator 2: Judgment Day
You will see several Hydrobots in action against Resistance fighters. You will see a scene when Connor and several more soldiers flying a helicopter. Connor yells "Do you see him?" to one of his men in the back, implying that maybe Marcus fell overboard. As he drops a line into the lake, a Hydrobot pops out and pulls the man in the water. He dies rather quickly while John is under attack with many of these things bursting through the cockpit to get him.
The Hydrobot may have been referenced in Terminator: The Sarah Connor Chronicles's "Today Is The Day, Part 1". During the episode the crew of the USS Jimmy Carter mention ASW Bots as being used to attack in the water.