Not sure what this is called, but on mine the rubber material is no longer there and there is basically an open hole. I assume it prevents water intrusion to the stern area by clamping around the voltage lines and steering lines. The pic is from Thataway as I forgot to grab a pic while on the boat.
https://www.dropbox.com/s/me7thid0gfxi7oc/gasketjpg.jpg?dl=0
https://www.dropbox.com/s/me7thid0gfxi7oc/gasketjpg.jpg?dl=0