Wherever to 15, planar sanctum once I got my first water form.
Kill the water mages there, since you have ABS it's pretty damn easy.
Can go into the water plane to for joke elementals that give decent xp.
You can do merians on the western aryth as well at a certain point.
Coral head once I had my second form (giant gar or something), was pretty easy to handle things.
With third form, bloated hippo, I went to the ruins of the deep and hero'd there, golems and sahuagins depending on what you have.
Here you may have trouble in that not all 3rd tier water forms can tank there.
All the final water forms can manage there pretty well, even the shark usually kills them so fast that they are not trouble.
I was killing the eye of the deep just for xp because it was so easy with that combo.
And people rarely screw with a water shifter underwater, which is nice.
Defense I got opposum, bobcat and komodo, never used any but the komodo though.
You could go to the coral palace and ship graveyard to rank as well, though the graveyard is cursed so you are sort of trapped if someone finds you there.