Hi Nicholas,
P mode is evil! Always shoot manual mode underwater. Here are the macro settings I use before I jump in:
1/125
f22
ISO 100
strobes on ttl
zero on ttl converter
On the 5D I like to use the * button for AF and set the shutter button to only do the shutter release, no AF. This makes manual focusing easier. This is my preference, others may like something else.
After you take a shot in ttl, the YS-110s will briefly light up green on the back as a ttl confirmation. You can also tell that the ttl system is working by noticing the preflashes coming from the strobes.
When testing the ttl system on the chihuahua fish, you can make sure the compensation is working by taking a shot at zero and increasing or decreasing the compensation dial. You should be able to see
Just remember that there may be times where the strobes are all full output and this may not be bright enough for a proper exposure. This will lead to underexposed images, although nothing is wrong with the ttl system.
Good luck with your shooting!
Jim