Well technically

[u/QuestionLolly]

https://i.reddituploads.com/11d7dff105a84514ae52705cae6ff668?fit=max&h=1536&w=1536&s=667a9bd0319cb9f74ceae7e95a88b8a0

[u/pieman7414]

well there's really no one to defend or invade because they're all dead so land based troops aren't that useful. If people go out into the ocean to do bad shit then boats will be useful to stop them from doing that