Giving the underwater terrors new homes.

[SangoProduction]

So, I can grant creatures legs, and the ability to breathe air.

What's the most horrifying / effective aquatic animal companion that is now available to me? I've got Moray Eel and Dolphin on my list right now for the top contenders, though Moray Eel is winning due to creep factor.

Level 4, but going up soon enough.

[Katie Boundary]

Scyllan from Stormwrack?

[ShurikVch]

At which level?

Say, Liopleurodon dinosaur from Dragon #318 is Colossal Animal with 38 HD... but required 24-level Druid
For a less "demanding" dinosaurs - Mosasaur and Plesiosaur (Stormwrack) are both required 19-level Druid
For even lower level - 16-level Druid get access to Archelon dinosaur, Dire Shark, and Giant Squid

But if you need 1st-level options, then:
Sea Snake, Medium Viper
Medium Shark
Snapping Turtle
Squid
Stingray