海月 (くらげ, kurage) – jellyfish

蟹 (かに, kani) – crab

海鼠 (なまこ, namako) – sea cucumber (Holothuroidea spp.)

海馬 (かいば, kaiba)
1. seahorse; sea horse
2. walrus

竜の落し子 (たつのおとしご, tatsu no oto shigo) – seahorse; sea horse
This is an interesting name for sea horse. It literally means ‘fallen child of the dragon’.

海豹 (あざらし, azarashi) – true seal (animal); earless seal

海驢 (あしか, ashika) – eared seal (esp. the California sea lion, Zalophus californianus); sea lion

鯨 (くじら, kujira) – whale (Cetacea spp.)

海豚 (いるか, iruka) – dolphin (or other small toothed whales, such as porpoises, belugas, etc.)

ドルフィン (dorufin) – dolphin

鮭 (さけ, sake) – salmon (Salmonidae spp.)

鱒 (ます, masu) – trout; sea trout