You don't commonly come across an "owner's manual" for a boat like you
do for an automobile. A new boat is typically delivered with a binder,
and in that binder will be individual manuals for the engines,
electronics, appliances, etc.

Odds are that many of the original systems on a 1990 Bayliner have been
replaced by now, so even if the original binder had been left aboard
much of the information would be obsolete.