Monsters:

Legendery Fisherman [given to Jounouchi]
Fortress Whale [also given to Jou]
Flying Fish
Amphibian Beast
Jellyfish
Great White
Fiend Kraken
Kairu-Shin

Magic/Trap Crads:

Umi (field magic card)
Torrential Tribute

Yes, I know Ryouta has more cards than that, but I'm trying to find out all of them! And that's hard work because Ryouta isn't easy to learn about!