Thou art like ... - Rather, Thou wouldest be like to (others, “wast likened unto”) a young lion.
And thou art - In contrast to what thou wouldest be.
A whale - Rather, crocodile (marginal reference note). Pharaoh should have been like the king of beasts, but he is a mere sea-monster. There is strong irony here, because the Egyptian king was proud of the comparison between himself and the mighty crocodile.
Seas - The word is often used of the waters of a great river, like the Nile.
Thou camest forth with thy rivers - Rather, thou didst burst forth in “thy rivers” as the crocodile does from the water into which he has plunged.