A trash bag will keep rain off a spare fleece for one trip. It will not survive a wet launch, repeated spray, or being sat on at the takeout. A roll-top dry bag closes by folding the fabric against itself, with no zipper to fail, which is why it still works on the fiftieth close.
The question usually shows up the night before somebody's first real paddle, and it is a fair one. Dry bags are not expensive, but neither is a box of contractor bags, and plenty of people have gotten away with the cheap version for a season.
So here is the honest version, including the part where the trash bag is fine.
When is a trash bag actually good enough?
Two cases, and we will name them plainly.
The first is a short flatwater paddle in warm weather where the worst outcome is a damp towel. If you are out for an hour on a calm lake in July and the only thing in the bag is a sweatshirt you do not need, a plastic bag under a bungee is not going to ruin your day.
The second is inside something else. A plastic liner inside a backpack, inside a hatch, is a real technique that experienced paddlers use for redundancy. It is a second layer, not the only layer.
Where does the trash bag fail?
Abrasion, first. Plastic film pinholes against a deck fitting, a seat rail, a buckle, or a rock at the launch, and a pinhole in a bag full of water is a bag full of water. You will usually not notice until you unpack.
Standing water, second. A canoe or a sit-on-top collects water in the bottom, and a bag lying in it is in contact with water for the entire trip rather than splashed for a second. Thin plastic loses that one quickly. Worth saying plainly: no roll-top is a submersion-rated container either, ours included, so the move on a boat that ships water is to clip the bag up off the floor rather than to trust any bag to sit in it.
Closure, third. A twisted-and-knotted plastic bag has no real seal, and it opens itself under load. A roll-top folded down three or more times and buckled holds its shape when a boat flips. The mechanics of that are in how to properly seal a dry bag, and the technique matters as much as the bag does.
And, less romantically, plastic bags tear, blow away, and end up in the river. A bag you keep for six years does not.
What about a ziplock for a phone?
A ziplock is a real answer for one trip and a bad answer as a system. It handles splash, it does nothing about pressure, and the zipper works loose in a pocket. If a phone is the whole problem you are solving, the full breakdown is in how to keep your phone dry on the water.
So what is the actual argument for buying one?
It is not that water is going to destroy your life. It is that a dry bag removes a small recurring decision. You stop rationing what you are willing to risk, you stop repacking at the launch, and you stop putting your car key somewhere clever and then forgetting where clever was.
A dry bag also solves the problem in reverse, which is the part nobody mentions in the buying guides: it holds the soaked pile on the way home so the car does not. That use is covered in the bag for the wet stuff.
If you paddle more than a handful of times a year, the bag pays for itself in not thinking about it.
Start with one bag, not four
Most people are well served by a single bag sized to how they actually paddle, and what size dry bag do you actually need walks through the liters honestly. Wild Currents makes roll-top dry bags in 2 liter, 10 liter, and 20 liter, in five prints named for real water. Water resistant, built for rain, spray, and wet launches. Browse the collection.
Frequently Asked Questions
Do I really need a dry bag for kayaking?
For anything longer than a short warm-weather paddle, yes. Sit-on-tops and canoes collect standing water, and a plastic bag lying in it pinholes and fails fast. A roll-top holds up to that kind of repeated contact far better, though no roll-top is a submersion-rated container, so clip it up off the floor.
Will a trash bag work as a dry bag?
For one calm, warm, short paddle where the worst case is a damp towel, it will usually hold. It fails on abrasion, standing water, and closure, and it has no real seal, so it opens itself under load.
Are dry bags worth the money?
If you paddle more than a few times a year, yes, mostly because it removes a recurring decision. You stop rationing what you are willing to risk and stop repacking at the launch.
Can I use a ziplock bag for my phone instead?
It works for one trip and fails as a system. It handles splash but not pressure, and the zipper works loose in a pocket. Fine as a backup layer, not as the only one.
What size dry bag should I start with?
One bag sized to how you actually paddle beats four bags bought at once. A small bag for phone, keys, and a snack covers most day paddles. A larger one makes sense if you carry a change of clothes.