I carry a large cheap laptop bag/briefcase combo for that. My friend has an alienware laptop backpack thats several years old, alienware is long gone but the bag has proven invaluable as a BAMF looking man purse/laptop bag with room for extras.
The maxpedition backpacks look similar to that function.
I'd look for something with 2 Big cushy shoulder straps. You want it to have all day comfort especially if you ever need to load it down.
Get the largest caliber you can fire accurately, rapidly, in the largest gun useful for the purpose.
My friend, look at the Camelbak lineup. I have the Ambush model and love it. Sounds like you need one with larger carry capacity, maybe the Hawg model. With most Camelbak models you will be able to store 3 liters of water conveniantly.
Democracy is two wolves and a lamb voting on what to have for lunch.
I took it with me to California in October. I had my DSLR with 55mm lense, 75-300mm telephoto lens and my wide angle. It also carried my Oakley ballistic sunglasses case, loose change the engagement ring I used to propose to my now fiancee. Not to mention a bunch of other stuff.
Right now... It has a 100rd pack of .22LR, 6rds of 20ga, windbreaker, poncho, flashlight, knife, altoids tin full of fire tinder, another altoids tin, reflecting mirror a cloth belt. There is still roof to stuff my Kel-Tec PF9 in there with a couple full mags and extra ammo. Not to mention enough room to conceal a full size XDm9.
If that bag doesn't work out for you Chris, check out the Booq bags (All Products by booq, laptop backpacks, laptop bags, iPad bags, sleeves, briefs). I bought a Booq bag 8 years ago to carry my work computer and gear in. I have used it EXTENSIVELY (almost every single day) for those 8 years and it is just now starting to show wear on one seam. The current bags are made of 1680D Ballistic Nylon exterior and 420D ripstop polyester linings.
If it does work out for you, keep us posted on it's performance and carrying capability...oh, and keep us posted on your trip to the beach.