Other then the most well known monsters, like the Kraken, there are many other sea creatures that were depicted on old sea maps that could exist below the depths of the seas.
It would be very cool to see more mythical creatures in the seas.
"Carta marina et descriptio septentrionalium terrarum (Latin for Marine map and description of the Northern lands;[1] commonly abbreviated Carta marina) is the first map of the Nordic countries to give details and place names, created by Swedish ecclesiastic Olaus Magnus and initially published in 1539. Only two earlier maps of the Nordic countries are known, those of Jacob Ziegler (Strasbourg, 1532) and Claudius Clavus (15th century)." -wiki
Map by Sebastian Munster, the era's most prolific mapmaker, famous for his great geographical work, Cosmographia (Basle, 1540). - RareMaps
Top L: "A woman holding seashell banner riding a merman with a trident, who together confront a woman holding a fish banner riding a merman holding a club, on Gastaldi's Dell'Universale of c. 1550 (British Library, Maps C 7.c.17)."
Top R: "A curious spiky sea monster on Gastaldi's Dell'Universale of c. 1550; this same monster appears in Andre' Thevet's Cosmographie of 1575 (British Library, Maps C.7.c.17)."
Bottom L: "A winged sea dragon with huge rabbit ears on Gastaldi's 'Cosmographica Universalis et Exatissima iuxta postremam neotericorum tradio[n]em' of c. 1561 (British Library, Maps C. 18.n.1)."
Bottom R: "A menacing sea monster on Gastaldi's map of Africa in the 1563 edition of Ramusio's 'Navigazioni', f. 261 (in British Library, G.6820)"