Following up on this, our Routing functionality that’s available in companion only creates serial and/or UDP endpoints on companion. Connecting to that from the topside requires making a UDP endpoint on companion, connected to the serial endpoint you want to connect to, and running a program on the topside that can communicate to that UDP endpoint.
Our docs provide examples for how to connect to the UDP endpoint options (client/server) using a python script, but we don’t have support/examples for making a connection present itself as a virtual serial point. To do that you would need routing functionality in a program on the topside computer, which you could perhaps achieve with a program like this 
It’s worth noting that the companion Routing functionality
Given your use-case of an imaging sonar, it’s quite likely you’ll want a higher than that 115200 baud rate, so the built in companion Routing functionality may not be your best option.
If you’d prefer not to use the linux tools mentioned in our docs, it may be possible to use
- virtualhere as per @etienne’s suggestion (one connection is free, I’m unsure of the data passthrough rates), or
- @patrickelectric’s open-source bridges (discussed a bit in the same post), which is already used in companion to pass through a 2M baud connection to/from a connected Ping360 scanning+imaging sonar