The backups are on a separate system with different credentials. One copy of the backups is sent to online storage that is immutable. You set a retention policy and then you can’t delete, overwrite, or change the backups.
Palo Alto would do what you want. PA410 or 420 would probably do for your ships. They’re not at all rated for harsh conditions, but they’re about as robust as you’ll find for basic network gear. If you get a PA for the home office as well, you can use their SDWAN for connecting everything.
For switching…how many ports do you need on each ship? I’m using Unifi industrial switches in our manufacturing plants. They stand up to the Texas summers in a highly alkaline environment. They’re only ten ports though (8 poe).
I have seen HDDs advertised as WI (write intensive), RI (read intensive), and MU (mixed use). The advertising says that the WI drives will last longer under write intensive loads. I don’t know how much truth there is to that.