Due to the rarity of their appearance and the fact that the only specimens washed ashore have been badly decomposed, there is a lot of scholarly debate about them. Most of it is contradictory.
A santhid is like a tiny, mobile island. It can be seen as a great shell sticking out of the water, crusted with seaweed and small crystals. Gnarled, with rippled and folded skin, it has long, drooping blue tendrils, like those of a jellyfish, only far thicker. These trail behind the beast deep enough within the water so as to be invisible.
It is beautiful, in a knotted, grey-blue mass sort of way beneath its shell. It seems ponderous, yet majestic, with mighty fins, and has two enormous eyes, one on each side of its head, surrounded by ancient-looking folds.
The one which Shallan investigates looks right at her and sees her inspection of itself.
They follow ships, and will wait for a ship outside a harbor until it resumes its travels. When the Wind's Pleasure is sunk after Shallan Soulcasts its bottom, the santhid that had been accompanying the ship rescues Shallan and deposits her on a nearby beach.