Don't sweat it!
Which ferry website are you looking at?
There is
www.bajaferries.com
and. www. ferrytmc.com
Two companies running a variety of vessels
and schedules, some on the short run to Topo'have no cabins because they
are not needed in a 7 hour trip.
.
Waiting to book a passage in person in La Paz is a perfectly
feasible option if you give yourself a day or two of cushion
in your route planning. Do not expect to have everything arranged on
a " timeline " expecting things to neatly fall into place.
They rarely will after a few days in Mexico.
The question about country of origin is a red herring.
Probably something for their insurance but if your dociments
are in order( TVIP,FMM, passports, etc) you cannot be denied,
you WILL get a booking.
With such a time cushion you can afford to relax a day in La Paz
or elect to take the alternate route to Topolobampo.
Either route is fine and the Topo ' run costs less.