The water line suggested location is shown in the owner's manual for most Boston Whaler boats
In the owner's manual, read the MAINTENANCE section. Read under the heading BOTTOM PAINT.
If you do not have a physical copy of the owner's manual, you can use my HTML copy that I published in the REFERENCE section of the website. CAUTION: the HTML manual is from the 1980's so the hull of your 1971 NAUSET 16 is not the same hull as shown in the c.1980 manual illustration. Your hull is probably the older non-Smirk design. But here are links to the c.1980 manual presentation:
https://continuouswave.com/whaler/reference/manual9-17/Follow the hyperlink to the Maintenance Section, then to the Bottom Paint instructions.
https://continuouswave.com/whaler/refer ... tml#bottomThe really best way for you to determine the static waterline of your particular boat will be to actually put the hull in the water with full rigging, with the typical gear aboard, with some fuel in the fuel tank, and then mark the waterline while the boat is afloat in dead calm water without any wind induced list or aberrant fore-aft trim. Mark the water line using good painter's tape that will stay on until the boat is back on the trailer.
Then use a laser level and align the fore and aft marks, and apply a marking or a tape line, recommended to allow for an additional two or three inches of deeper draft.
You should read the owner's manual regarding what needs to be done before the anti-fouling paint is applied, that is, you should apply a two-part epoxy paint to the areas of the hull expected to immersed in the water to prevent ingress of water into the laminate and possible osmotic blistering.