Hi shipwright, standard digital radio gear as fitted to model boats will do the job,. I am unsure of the frequency used for model boats in NZ, you will need to find out, but it differs from the model aircraft frequencies so don't buy a set using aircraft frequencies, however,the equipment is the same and it does not have to be anything special. You cannot use the newer 2.4 gHz radio gear, the frequency is too high and will not penetrate through water.
Standard digital radio equipment on the marine frequency will happilly penetrate fresh water with the subs aerial inside the waterproof unit whilst submerged. It is all useless in a salt water environment as none of the signal will penetrate this medium, in this case, you would have to carry your aerial out of the boat so it was visible at the surface when the boat is submerged.