I want to replace the Lowrance GPS/depth finder on my boat. The depth
finder uses a thru-the-hull transducer and that will be replaced also.
The diameter of the transducer is 2". Is this a standard diameter used
by all GPS/depth finder vendors. If it's not standard, which vendors
offer transducers with 2" diameters?

No, 2" diameter is not standard, although lots of manufactures use
Airmar transducers with 2" diameter.
Airmar and another quite common transducer manufacturer, Radarsonics, both
have web sites with more
information about size, shape, function and compatibility than you can shake
a stick at. Radarsonics, for example has some
really great bronze body transducers with 3" diameter, and some plastic ones
with 2 1/4" diameter.
I wish I had more specific information, but do not.

Old Chief Lynn