Im assuming a fast planing hull style boat here
Two motors definitely requires 2 controllers for Brush-less motors, they can be run off one channel with a Y servo cable, you cannot save here. A 30 amp unit per motor should be enough but I would probably go larger just so you are not pushing the edge.
As you have specified the voltage it gives a fair Idea of the Kv or RPM per volt the motor needs to be and for a fast boat 10,000 RPM is a good starting point, lower if you use a larger prop or want to sacrifice performance for run time.
On 8.4 volt an 1200 Kv motor to achieve 10,000 RPM or 1050 Kv for 9.6 give or take 10% and this gives you one important number, the size of the prop will determine the can size, and for anything close to 40-50 mm
(which a 46 inch boat would probably use) a 36 mm diameter out runner will prove sufficient.
The motor will probably not need cooling if you keep the motors to what I recommend, a lower Kv motor will reduce the possibility even more. For the relatively short runs the Nimh will give, a fan cooled Car ESC will work. If you run parallel packs then using a water cooled ESC would be better due to the longer run time adding more heat.
As for prices local members can point you out to good UK sources, I buy my gear from Hobby King out of Hong Kong.
Cheers
Nick
Another option to consider is run the 2 packs in series (both packs supply both motors and double voltage) and halve the Kv of the motors to 5-600 Kv, this increases efficiency sllightly but ads a little extra complexity to the wiring.