Do you mean a Pro 50 vehicle unit or a pro 50 water station, the Water station is the only one approved for marine use and available in 24v only? Personally I would go pressurised, we fit almost exclusively these days, never fit thermostatic valves as the induce the boiler to cycle, not a good thing, the best thing you can do is have a good read of the Webasto marine install manual, it will answer all your questions and some you may not even have thought of yet. If you drop an email to ecmhe@talktalk.net I will ping you back a copy.