One the common mistakes newcomers to paddlesports make is overstuffing their dry bags and not leaving enough space to really roll the bags closed. Take the time to squeeze out any extra air in the bag before you start the whole the whole buckling process.
Many dry bag manufacturers recommend a minimum of 3 complete rolls to provide the closure for the bag, i.e. 4 or 5 complete rolls is quite acceptable. A dry bag offers a level of water resistance but they are not intended for long term submersion.
I found this out the hard way during an adventure race near Muskegon, Michigan. Attempting be be slick, I put a small lightweight dry bag inside my race backpack to protect my stuff under any conditions, or so I thought. During one of the swim sections, I got tired and flipped over on my back for a while. The dry bag was entirely submerged soaking my medical kit, clothing, spare maps and other vital items.
Dry bags are not 100% effective for everything you can imagine. Items can be crushed if mishandled. Disclaimers abound regarding suitability for electronics, optics, etc. Your beloved equipment can suffer permanent damage or failure. A capsized boat, bad kayak hatch closure, compartment pinhole, or other incident may force the dry bag to be covered with water.
Dry bags will also wear out over time depending on the abuse you dish out. A little sand in a kayak hatch or the bottom of a canoe can eventually rub a pinhole in the bag. Unloading all your gear onto rocks, rough concrete or asphalt takes it toll over time. Leaving a dry bag out in the sun to dry exposes it to ultraviolet rays. All the above and more may hinder the ability of a dry bag to protect your stuff.
Packing a spare set of clothing as a redundant contingency plan is always a smart idea. So is checking your paddling gear every once in a while to make sure it is free from defects. Repair and replace items as necessary. Take the precautions to insure your survival.
I'm often asked why I throw my cell phone and keys in a small hard shelled dry box which I then place into a dry bag, which then goes into my kayak hatch. I smile as I answer "experience over the years has taught me well." Items under a bungee can be swept away by waves and lanyards can fail.