Its always best to mix it in very small batches for every part needed as it will harden fast.
For doing it under water, 1st shut off all running pumps, have your rock work the way you want it and make sure to know exsactly where you are placing the FijiCrete.
Now for mixing FijiCrete very little water mix fast to the consistency of almost thick putty, turn it into a ball shape as it really shouldn't stick to your hands then slowly squish it into the spot that its needed (don't start to play with it) and 10 minutes later turn pumps back online.
Don't start to poke at it(something I have done in the past
) as the 1st layer might dissolve but it will be there for good holding the rock-work in place.
if the pumps are hitting right against the FijiCrete then make sure that specific pump is off line for a little longer than 10 minutes.