I have two fathom x and a raspberry pi, and ubuntu computer. I just wanted to send a sample data and see if i can get information across easily. Is there a software that i can use to test this or some kind of way of testing this idea?
You can try ping.
Get both computers on the same network, then type
ping <other computer's ip address> in a terminal.
So i can have my computer connected to the first fathom x and then the first fathom x connected to the second fathom x. I will then connect the second fathom x to the raspberry. When I do this, I then can use the ping command on my computer to test the connection to pi right?
Yes, but you will also have to configure both of the computers’ ethernet interfaces to be on the same network. Look online for setting up static ips. For example, this is how we set up our ROVs: https://www.ardusub.com/getting-started/installation.html#network-setup.