This is VERY true! There were 4 of us on our last cruise, and we spent the majority of our time by the pool. We gave the server a $20 tip on the first day, and he took very good care of us the entire rest of the cruise - would even come from the opposite end of the theater when he saw us come in for a show and be the first one to greet us. He would check on us regularly, but not to where it was annoying, we never had to wait more than about 5-ish minutes for a drink once it was ordered, and he regularly brought me ice for my water bottle without me ever asking for it (I was fine without the ice, but it was thoughtful and as it melted, I had to get up less often for refills).
I also added an additional $1-2 for each drink I ordered, and we gave him a little extra again at the end of the cruise because he just went above and beyond for us. I think with 1 exception (and outside the dining room), we only ordered drinks from him the entire cruise.
We did the same with a bartender at the atrium bar on a different cruise a few years back, and not only did the drinks get significantly better, but even if the bar was totally slammed, she would come to us first when she saw us.
That said, I have still had good service without giving extra tips, and with 18% already added on, you shouldn't feel any guilt about not adding extra if you don't want to.