Crater seas take elements from three major sources: space (if they are an impact crater), the ocean, and the coast. Effectively combining elements from these sources can generate believable and interesting crater seas. For instance, the seas contained within the crater may represent an unusual environment, especially if the asteroid that made impact contained strange fictional material (or stranger beasts from across the cosmos).

“And the severed head of the pig god, Olthmus, fell from the heavens to the earth. The impact was so fierce the world shook for three days and three nights, and then only the crater remained…”