I would be reluctant to sand the shinyl-vinyl faux, woodgrained bulkheads and paneling, since the resulting texture would be rough and inconsistent. But where preparation takes up 75% of painting prep on normal projects (wood base), I'd say your painting prep requires 90%. Applying the final coat of paint is the easy part.
First remove all detachable hardware, fixtures, devices and fittings and clean the vertical surfaces very well with a degreaser solution. Dry the surface well and then apply a shellac base primer - such as Zar or B-I-N, using a fine bristle, compatible brush.
After drying - typically fairly quickly, I would lightly sand the surface with 240 grit to remove any surface particles which may have been suspended in the primer, followed by dust removal.
Lastly, apply at least 2 coats of a good quality enamel paint - preferably no glossier than eggshell, or satin finish. You should avoid reflective finishes on older, irregular surfaces - all defects will be amplified. I would also avoid white, instead choosing a soft, neutral and warmer hue of buff, or tan. Hope this helps.
True Blue . . .
sold the Nauticat