Parts list and plans for high-power waterproof thermo controller

I need a qualified industrial controls or related engineer to tell me the right way to do this. I need a complete specification of the correct off-the-shelf parts that I can order in the United States and hand to an electrician to wire together. I am not an engineer or an electrician. You must provide a specific parts list I can order from, with prices from a United States vendor, and an adequate plan I can hand to a tradesman to carry out. I have a large (750 L) saltwater aquarium heated with a 220 V, 30 A inline heater in the water circulation system. This heater's built-in analog thermostat in the is not accurate or adjustable enough for our needs (we need +/- 0.2 C at 35 C). I'd like to supersede it by wiring a power relay in series with the heater's power supply, and using an appropriate digital thermo controller to turn it on and off. The heater operates in a damp environment exposed to saltwater splashes, and its electrical controls *must be safe*. The fewer parts we need to assemble, the better. The heater operates at ~6 kW so most cheap thermo controllers are not adequate, but I become lost as soon as I start trying to look at real industrial controls catalogs. The heater model is: Coates 6ILS, [login to view URL] Please respond with your qualifications and an estimate of your time required, and confirm your understanding of what the work product will be.