This just ain't true at all. Carnival has a drink package that cuts you off at 15 drinks. But if you don't have the package you can purchase as much alcohol as you want. It would also be a bytch for them to track as people will buy drinks for their table or shots for friends.
I didn't make it to 30 but back when I was a really bad binge drinker I had over 20 drinks in a night. I had ordered the drink package, but for some reason it wasn't being recognized in the system. I went to customer service and they told me they'll look in to it and to go ahead and order drinks and they'll just comp them off my account. Got out partying, and doing shots all night. Snapped out of it while on the top deck in the middle of the night making out with a woman. The next morning I counted my drinks and had ordered 23.