There are loads of water pumps, more than capable of the pressure needed for water injection about, ie a bog standard screen washer pump from some cars, or headlight washer pump, std mondeo one about £8 from Halfords!!
In the traditional fuel pump fitted on cossie etc, the whole inside of the pump complete with electrical connector points has petrol flowing through it, but with water brings oxygen/moisture and so corrosion occurs.
tabetha