need to see source IP when using SNAT for ssh connections

Your server must be in a network attached to the BIG-IP. Then its the floating IP of the egress network when the traffic leaves the F5 on its way to your server.

For example if your server was in the 172.31.10.0 network and the F5 had addresses of 172.31.10.5 (traffic-group-local-only) and 172.31.10.6 (traffic-group-1) then you would use 172.31.10.6 as the default route for your SSH server.