Escape Submersible is an object belonging to the Arachnos faction. It has Lieutenant class hit points and no ranged nor melee attacks, nor it can move. This makes this item an easy target to destroy.
The mission briefing suggests that Night Widow Nocturne will try to reach it (which would make you fail the mission), so destroying the sub before engaging Nocturne might be a good idea.
Three of these subs (under the name "Arachnos Mini-Sub" this time) can also be found during the mission Stop Arbiter Sands and get the PCM form Agent G. The mission suggests that Sands could try to run to them and escape. so I did with them the same as I did with Nocturne's one
Arachnos uses small submarines like this to sneak operatives into and out of Paragon City undetected.