A sea spider is a spider that dwells under water. The only physical appearance of a sea spider is in "Patrick's Coupon" where Patrick encounters one.
The sea spider looks similar to sea urchins. In "Patrick's Coupon," they look very similar to real Spiders with a grayish body and red eyes. The spider's legs are hairy and are extended to suit the body. They also have a gland on their abdomen to spin out webs. It should also be noted that Sea Spiders have monstrous teeth, most likely meant for their prey and predators.