Yup, it's background radiation on the Internet. We all get them.
If SSH key authentication only isn't enabled, turn it on. Change the port sshd is listening on.
Set up fail2ban to further protect the new port (I get a lot of portscans hammering my nodes
looking for the new sshd port followed by brute force attempts, so may as well cut 'em off
at the knees).
Or set up a hidden service for sshd on the box and reconfigure it to listen on the loopback only.
You'll only be able to SSH in over the Tor network after that, but it'll cut the login attempts way
down.