Weedy seadragons are not good swimmers because their bodies are surrounded by protective dermal plates and they lack a caudal fin.
Weedy seadragons are not sessile or immobile, but they are not very good swimmers, either. This is because their bodies are surrounded by protective dermal plates, which inhibit their mobility. Also, they lack a caudal fin, and therefore must rely on their ventral and dorsal fins for swimming.
Weedy seadragons do, however, have a long dorsal fin along the back and small pectoral fins on either side of the neck. These fins help provide balance.
Because they are poor swimmers, each year a number of individuals are found washed ashore on the beaches of southern Australia.