Rahab

Castlevania: Dawn of Sorrow
"A monster that dominates the seas."

Castlevania: Dawn of Sorrow
Resistant against water

Castlevania: Dawn of Sorrow
Weak against stab attack, lightning, and light.

Castlevania: Dawn of Sorrow

 * 1/1 Level
 * 0 MP/Use
 * "Gain the ability to move underwater."

Strategy
Recommended Level: 25 Seal Level: 3 Soul: [A] Allows you to walk underwater

-Soma Strategy

Not too hard of a boss, either. Wait for the fish-snake-eel thing to jump up into the air; that's your chance to attack. When it comes up out of the water, it'll do one of 3 things. The first is jump over you, Free Willy-style. It leaves behind a couple of ice spikes that fall down. Swim between them to avoid getting hit. It 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 it'll do is spin around and break all of the ice platforms it's made. Lastly, it'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.

-Julius Strategy

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 it descends you'll be hit. You can also follow it when it 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.