Like Jim, I think that cruiseship security is very tight.
As passengers we don't see everything they do. I do know that both checked and carry-on bags are screened (not just for booze!). Those cute little beagles aren't there to entertain us (even though they are VERY cute!) There are scuba divers around the ship at embarcation and probably at other times. None of this is a secret.
I'm sure there are many more security measures (that are secret) in place that we don't know about for good reason! Just because you don't actually see what's going on with security doesn't mean they aren't doing their job.
Personally, I feel safer on a ship than on an airplane, or even in a car!