Totally agree, using ships water would be very unwise.
Make sure if you are going to go this way that you work your levels out so that salt water can never get into the fresh tanks
Which is the reason for a non-return valve tap-type fitting where we can only connect the hose. Using our freshwater filler hose (from mains water) was the intention. We don't have a way of connecting a water hose from
the freshwater tanks on the yacht.
First plumbing salt water to fresh water even with a non return valve is a bit of a no-no
Having said this, there must be a way around it as most 'auto-flush' desalinators use ships freshwater into the saltwater line via a 'T' with no non return valve. Here they rely on power (or loss of power actually), to close the freshwater-flush-valve. The point you raised of using all the ships freshwater would be a concern here too if the valve did not close.
Not sure how they get around that challenge. Any thoughts?