The Sea Worm cant actually be spawned in through any summoning items like the Eye of Cthulhu can with the Suspicious Eye. It will only spawn in between 4:30 AM and 6:00 AM, and will only spawn at the beach. They have a fairly high spawn rate, but will only spawn once the Eye of Cthulhu is killed.
The Sea Worm will most likely the second or third boss you run into while playing.
The Sea Worms texture is only a place holder, and when in water, the body parts look displaced.