Dry bags come in a variety of shapes, sizes, colors, and levels of dryness.
'Splash resistant' means that contents will stay dry from small splashes. If the bag is submerged, the contents will get wet. Great for soft coolers and beach towels as well as other items that won't be ruined if they do get wet.
'Waterproof' is a label that most drybags carry. The material is in itself waterproof, but if the bag is submerged, then the contents may or may not get wet, depending on the closure system of the particular bag. Most people associate this style of bag as the norm for whitewater as they are readily available and moderately priced. Waterproof drybags typically have a 'roll down' closure system that you want to make sure that the roll is verey tight as to maximize dryness.
'Submersible' drybags are as advertised. Contents will stay dry as long as the seasl is not broken. These bags typically cost more money, but are of infinite value if you rely on your gear (clothing, camera, otherwise) HAS to stay dry. These bags tend to last a lifetime of use and abuse. Well worth the money in the long run.