What are the total current number of recreational boaters,(sailboats
and motorboats), on the East Coast of the U.S., that have p.c.
computers on board?

Considering that Every sailboat and Motorboat that is used/can be used
for over night is required to have a radio (which techinicly is a
computer)by the coastgaurd. it should be %100 of seaworthy vehicle's.

But since you are asking about a computer as in a personal computer.
The answer may never come because no-one ask's were the computer is
going to. Therefore short of having a complete census of the east
coast or checking number of sales of conversion/specialty parts on the
east coast, there is no way of finding out.

This is an "easy" answer, and is made without doing any research. Of
course there is no single way to reductively conclude a specific
number.
But there are: 
sales of "marine" notebooks, made by specific notebook manufacturers,

(continued)- there are sales of installed p.c.s wired directly into
nav stations (by boatbuilders), navigation software sold to boat
owners (by at least 4-5 major marine software companies, etc. I was
hoping that someone had already gone thru this research and compiled a
range which would be a reasonable estimate.

Stating that a vhf radio is a computer is - ??.
The computer's found on sailboats are usually associated with
Navigation, and Plotting Software, and would most likely be found on
boats cruising beyond their local waters - therefore boats over 34'
would be "most likely" to have them. The bigger the boat, the longer
the cruise, the more likely. But nevertheless - what is the
approximate number +/-?