I'd leave the beer out of the bag... especially if you're doing any night sailing, since alcohol interferes with night vision, balance, judgement, etc.
Are you talking about what people carry in their crew kits? It really depends on when/where you're sailing. It also depends on what kind of sailing we're talking about—what you might want for a daysail around the harbor is going to be a bit different than what you'd want on an evening race or a short delivery.
In New England, it might have a watch cap, 3/4 season gloves, polar fleece pullover, anti-nausea meds, hand-bearing compass
, handheld GPS
, digital camera, iPod or music player, some snacks—I prefer Snickers bars, something to drink—I usually pack Gatorade, and possibly a change of clothes. Often, I'll bring my own Type V PFD, harness
, tether, and foul weather gear. A rigging
knife, pocket tool, sunscreen, hat, sunglasses, all might be good additions, depending on the season and such.
The bag I use is a backpack, and a lot of the things are stowed inside a small drybag inside the backpack. This helps keep the stuff organized, and also protects it from spray, rain, etc. I've found that the terrestrial idea of what is waterproof, and the mariner's version of what is waterproof are significantly different.