I know dry bags come in different sizes and materials, some being soft and some being rigid and tough. However I've noticed there are items in the market that are exactly the same as a soft dry bag but are called 'sac liner' or 'rucksack liner'. I can't spot any differences as the closure is the same and all the seams are sealed. So I'm wondering, can I use a sac liner as a dry bag to carry in a small kayak?
The soft dry bags aren't as good as the stiffer sort anyway. My traditional plasticised dry bag can handle long periods in the water if it's been rolled down properly, but my nylon ones eventually let a little moisture in if squashed around in water. I think the roll seal isn't as good if they're too flexible, especially if you also attach it by the loop formed by clipping it shut. Also the dynamic pressure of moving water could easily exceed the hydrostatic head of many fabrics. So I'd be very wary about using a large soft one for things that have to be completely dry. My first aid kit, for example, is in one small dry bag inside another which it shares with other emergency kit that can handle a little damp.
The soft dry bags sold for watersports use have some reinforcement to help the top roll down properly (even though I've expressed my doubts about how well this works). The ones sold as liners often don't.
Rucksack liners don't require as much sturdiness as a dry bag since they go inside a rucksack/backpack, which gives it protection and structure. As such rucksack liners weigh less than dry bags and alone won't stand up to the same abuse but inside a backpack will perform optimally.