I had to check to see if you lived in the United States or a country where tipping isn't the norm.
Nope. Ohio.
You are absolutely right I, and many others, tip additional amounts to gain benefits like faster service or longer pours. I'm not ashamed to acknowledge this.
On a cruise ship (or even an open bar at a catered event), a noticable tip is definitely a bribe for the next drink. The $20-$40 I give the cabin steward on enbarkation day, is intended to be remembered.
Whereas, when I go to a regular restaurant bar, or even a frequently visited coffee shop, a larger than 20% tip is both a thank you for those extra long pours, or friendly service in the past, and also for my next visit. I like being treated above and beyond. You treat me above and beyond, and I will treat you above and beyond right back.
It's an "I scratch your back, you scratch mine" tipping culture I live in. As long as I live where tipping is presumed to be acceptable, I will do it willingly.
If you're ok with being treated average and unrecognized, then by all means, the included tip on your cruise is fine.