Difficult to give a recommendation for something like this. Perhaps some of the people with U47's could chip in. A few extra bits of information would help.
What props are you running- diameter and pitch?
What speed do you want the boat to run at? As standard the U47 is quite nippy, as the boat is designed as a dynamic diver. You don't need that speed if statically diving, but some people prefer a fast boat.
Regarding hole sizes, that will limit your choice of motor. I would consider making some adapter plates, as brushless motors can be much smaller than they're brushed counterparts. You'll probably want some new couplings too, as most brushless motors suitable for your tub will be 3 or 4mm shaft diameter.
My thoughts are that a pair of 28-30mm outrunners rated somewhere between 700-1000kv will be about right. I find Hobbyking excellent for inexpensive brushless motors and controllers.