The Aquatic Twitcher (called sewmo in the files) dwells in the sewers of the city and can be seen lying down in the water, where they await for unsuspecting victims to pass by. Their primary attack (while tied up) is to shoot their tongue, which reaches long distances. It can easily be dodged. Their movement is restricted when they first spawn since the twitcher's arms are caught in barbed wire. The barbed wire can be broken from repeated strikes from a melee weapon, but it is not recommended to do so because the twitcher becomes faster and more powerful. Hitting or shooting the creature in the head can potentially prevent the creature from getting loose.
Relevance to Simon is that it could be the murdered victims of Doctor Purnell , being tied up and thrown into a sewer to drown. The way it attacks when it is caught in barbed wire may slightly represent Simon's anger and bitterness by "lashing" out.