I investigated the beer selection while I was onboard during our 5/26 cruise. I was maintaining a thread entitled "Let me answer your questions, 'live' from the Magic" (<a href="http://www.disboards.com/showthread.php?threadid=38440">link</a>) and two folks asked me if they have Guinness on board.
Here's a complete list of the beers available onboard:
<table border=0 cellpadding=5><tr><td valign=top>On tap</td><td><nl><li>Budweiser</li><li>Miller Lite</li><li>Heineken</li></nl></td></tr><tr><td valign=top>In cans</td><td><nl><li>Beck's</li><li>Budweiser</li><li>Bud Lite</li><li>Coors Light</li><li>Guinness</li><li>Heineken</li><li>Ice House</li><li>Miller Genuine Draft</li><li>Miller Lite</li></nl></td></tr><tr><td valign=top>In bottles</td><td><nl><li>Amstell Light</li><li>Bass</li><li>Corona</li></nl></td></tr></table>
These are the beers available at most full-service bars. They also serve the draft selections at various stands set up on Decks 9 and 10 during the day.
Draft Beer (16 ounces) costs $3.25 for domestic and $4.00 for imported.
Canned / Bottled Beer costs $3.25 for domestic and $3.75 for imported.
There is a refillable mug deal where you can get your 22-ounce mug refilled with one of the draft selections for the price of 16-ounces. When I sailed, it was a heavy glass mug that I wouldn't have wanted to carry around. Recent cruisers have reported that the beer mug is now plastic.
Keep in mind that there is an automatic 15% gratuity added to your bill for each one.