The only source of a wiring diagram for a c.1997 Boston Whaler boat of a particular length and model would be Boston Whaler. If you contact Boston Whaler customer service they may have something for you. That said, and as you mention, the instrument panel wiring on a small boat like a 15-footer is generally not particularly complicated, and it would not surprise me to learn there was no particular schematic diagram ever provided to customers.
A further problem with a boat made in 1997 is that it has been out from the factory and in service for 21-years, and there is no guarantee or even much of a chance that the wiring remains exactly as it was when the boat left the factory. Boats have a way of getting modified, and in the case of wiring the results are often not very pretty nor especially well done. Of course, that's not to say that when you get done re-wiring the installation won't be better than new. :-)
For general advice on electrical installations on a small boat, see my article in the REFERENCE section:
Boat Electrical Circuits and Wiring Practiceshttp://continuouswave.com/whaler/reference/boatWiring.htmlThis will give you a good foundation for installation of electrical systems on your small boat.
For information about the ignition wiring of OMC motors in particular, see my article in the REFERENCE section:
Ignition Switch Wiringhttp://continuouswave.com/whaler/reference/ignitionSwitch.htmlOMC harness usually use these colors and terminal designators:
I or IGNITION = Violet; switched 12-volts from ignition key
G or GROUND = Black; battery negative
S or SENSOR = Gray; from engine charging circuit
If a gauge has a lighting circuit, it will be marked
L or LAMP = Blue; gauge illumination circuit
Here is a list of the various color codes for wiring:
http://continuouswave.com/whaler/reference/wireColorCode.htmlBefore you disassemble any wiring, make an accurate sketch or diagram of the existing wiring and with notes about wire insulation color. Also take some close-up pictures to show details at switches and junctions for future reference. A dozen good pictures might save you a lot of trouble later.
Also refer to this prior discussion on a similar topic:
viewtopic.php?t=120You will find a schematic diagram from Boston Whaler that is mostly a generic drawing of what you might typically find:
Nota bene: the above drawing is listed in my collection of DRAWINGS AVAILABLE in the REFERENCE section article about such drawings and look under the category "Electrical". See:
http://continuouswave.com/whaler/reference/drawings.htmlBoston Whaler's owner's manuals also gave a narrative description of the typical boat wiring. You can find this in my reproduction in HTML of a Boston Whaler boat owner's manual. See
http://continuouswave.com/whaler/reference/manual9-17/electrical.html