This will have a subjective resolution, but the things I will be looking for are:
Direct comparisons to 9/11 by major international news organizations,

Featuring at least one major terrorist attack using a ship,

And evidence of at least ~10% of the death or property damage directly inflicted by actual 9/11.

This market is inspired by the rising sense of vulnerability to international business represented by shipping logistics and related infrastructure. In particular, this market is focusing on the possibility of state or non-state terrorist actors using ships as improvised weapons in a method broadly similar to how planes were used on 9/11.

A warship being used by a military to launch missiles would not count, even if it has blatantly civilian targets. However, a hijacked ship being ran aground and killing 300 people would count, even if it doesn't hit a particularly huge building. Further clarifications can be made in the comments, which I recommend potential bettors read.