I'm sure most of that would apply to Digital Ocean.

Also, here's a good guide to iptables: https://wiki.archlinux.org/index.php/Iptables

In general, if you've got all ports shut down except those you need

and
ssh is restricted to key-only login (and definitely disallow root
login!) then you'll be in a good place.

Obviously, you can take security to the nth degree but the main attack
points will be through the software you're intentionally exposing (web
applications) and for that... good luck :)

btw, I'm not a security expert ;) Others on the list might be. I take my
cue from the IRC channel: "advice given here generally isn't".