I know you already have your STW600 and had your heart set on running it from the inverter. I just went through a project reconfiguring an old boat with an old school electronics guy and he was very down on the use of inverters because of the inefficiencies in converting power and when you do the math, most boats do not have nearly enough batteries to do what we think we want to do. And stay over 50% charge to not kill the life of the batteries. So good choice to not run your WM from the inverter. One thought I had regarding the starting of the high pressure pump, would it work to just install a T in the pressure line with an extra ball valve that you could open for starting and then slowly close it to bring it up to pressure? Vent the leaked water wherever is handy.
My main thought for you is to use a smaller DC WM that only draws 10 amps at 12 Volts and is rated at 8 gal/hr. If you do your 8 hr cruise you could fill your 144 gal water tank from the main engine charging system and also have your battery bank charged at the end of the day. With that low of a draw you could make water daily from solar power and not run any engine. I found these Spectra WMs online and the Ventura 200T is the one the specs are for. List is $6,795. They have smaller ones too, about 4 g/hr for $4,000ish. Does anyone else have any experience with these DC units? Maybe OLD DAN will chime in!
Just a note on why your 1 1/2 HP single phase motor will not start easily is these single phase motors, specially on 120 volts, have a current in rush of at least 7-8 times full load running amps. Specs on a Baldor 1 1/2 HP 1800 rpm capacitor start motor are full load running amps 6.2 amps at 230 V, 12.4 amps at 115V which is what you had from the inverter, and 7.7 times full load amps at start. So now you have 12.4 A x 7.7 = 95 amps trying to supply power down a #10 or #8 wire so your voltage drop is very bad. Take into account the hundreds of feet of wire in the marina supply system that is rated for a 30 amp service and you have a severe overload and low voltage which spikes the amps even higher by Ohm's law. I wondered if your WM was possibly belt driven maybe you could have changed pulleys to slow it down, use a smaller motor, like 3/4 HP, that came more in line with the supply of power you have?