A great day backpack for your trip to the beach is this Quechua NH100 20L Backpack.  There's a single large compartment, dimensions are 18.5" H x 9.5" W x 5.5" D and a volume of 20 liters,  with a divider and an internal zipper pocket, a zipper pocket accessible from the outside as well as two (2) bottle holders.  For easy carrying, the back has  two (2) foam pads, two (2) padded straps, a chest strap and two (2) thump loops.

Each backpack is abrasion resistant and durable, coated with 100% Polyurethane, with a ten (10) year warranty from the manufacturer.  The main fabric and lining are 100% Polyester Cationic and the webbing is 100% Polypropolene.  The backpack's carbon footprint is lowered by minimizing the amount of dyeing required.  These Quechua backpacks really are a great value. 

And for those interested in the name Quechua, it refers to the aboriginal people of South America, primarily from Peru but also smaller populations in Argentina, Bolivia, Chile, Columbia and Ecuador.