Major Mobs can attack people in submarines from outside the submarine

[GusHobbleton]

As the title says, enemies outside the submarine can affect you when you're inside the submarine as if it isn't there.

[Berrik]

This also applies to lockers and most any other kind of container you can hide in.

[Technature]

Pods too.
Someone once got headspidered because they reasonably believed they were safe from one inside a space pod.

[Triacontakai]

Very longstanding issue, apparently it's hard to fix or something? Not sure about the specifics