A massive sea serpent, Rahab is named for a Hebrew water demon normally associated with Egypt and the Red Sea. He is mentioned in Psalm 89:9-10 and Psalm 51:9-10. This is different from Rahab, the Jericho woman in the book of Joshua, although their names probably share the same etymology.
In Castlevania, Rahab is an icy Water Dragon. You generally fight on a surface of ice, which can be broken or melt.
|Enemy Data: Rahab|
|Image||Name - Game|
|'|| 4. Ice|
|A monster that dominates the seas.|| Strong: Water|
Weak: Lightning, Spears
| Soul: Gain the ability to move underwater.|
Dawn of Sorrow Strategy
Recommended Level: 25 Seal Level: 3
Soul: [A] Allows you to walk underwater
Not too hard of a boss, either. Wait for the fish-snake-eel   guy to jump up into the air; that's your chance to attack. When he comes up out of the water, he'll do one of 3 things. The first is jump over you, Free Willy-style. He leaves behind a couple of ice spikes that fall down. Swim between them to avoid getting hit. He should stall a few seconds under you, where you can also jump up in the air and slash after you enter the water. The second thing he'll do his spin around and break all of the ice platforms he's made. Lastly, he'll make ice platforms that you can stand on. But don't. It's better to float in the water and avoid being hurt when he breaks the platforms.
This plays a bit differently since the trio automatically has the ability to walk underwater. Just stay under him and keep attacking. When he swims to the bottom of the pool, duck down and attack. If you're standing up while he descends you'll be hit. You can also follow him when he jumps up out of the water and land a few blows on him, but you'll need to quickly get to the bottom of the pool after. You could also just keep double-jumping, but I found crouching at the bottom to be easier.