AstroDad:
Making the cable will be the easy part. As Bill mentioned, it's just a USB to RS232 Serial cable with a custom connector for the dive computer (the RS232 serial side of the link). I have a Genesis ReAct Pro and it has three contacts that mate to the RS232 side - Tx, Rx and ground - odds are you have the same setup. Grab a break-out box and you can map the pinout of these connectors.
But then what do you DO with the cable? If you don't buy the "kit" you don't have the software from the vendor to download the data to your PC. Are you going to use a 3'rd party product? Are you going to develop your own? If the latter, you're going to need something like this
www.aggsoft.com/serial-port-monitor to reverse engineer the API - not a trivial task.
Actually, you wouldn't need to reverse engineer the serial
serial data message interface or write your own s/w.
Aeris/Oceanic give away their s/w for free. A version
comes with your dive computer on CD and you
can download the latest versions from their websites.
One challenge would be to figure out what their s/w looks
for to find the COM interface. Assuming, you could figure that
one out, then you could make your own cable and "trick" the
s/w into using it.
Another challenge would be to figure out the voltage levels
used by the dive computer on its serial port pins. I haven't
measured it but I'd bet its not your typical +12/-12v "standard"
electrical level serial interface.
The third and perhaps one of the most difficult challenges would
be to figure out how to make a connector that would mechanically
attach to the dive computer. The Aeris/Oceanic connector
is pretty complex. It loosely snaps into the dive computer
and three spring loaded prongs push against 3 small rods
that are perpendicular to the spring loaded prongs.
While not impossible, this seems very difficult to come up with
something that would work.
So given enough time and access to a factory Aeris/Oceanic
cable you could probabaly do it.
But for $49, I sure wouldn't mess with trying to figure all
this out and make one.
--- bill