The traditional, most common design for waterproof bags is for them to have a roll seal which doubles as a handle to carry the bag with, with no other straps or other method to carry it. Whilst this design may ensure the bag is as minimalist as possible, it really limits the amount of use you can get out of it. There are four reasons why the best and most useful waterproof dry bags have a shoulder strap to carry it with:

You can do much more with them.
They can be secured to things more easily.
They make life easier.
You get the best of both worlds.
Let's look at each in turn and explain why the next waterproof bag you purchase should definitely have a shoulder strap.

1. You can do more with them

When you have a shoulder strap and your waterproof dry bag is on your back, your hands are free. This means your bag can accompany you on any activity where your hands are needed, and your gear needs to be kept dry. This opens up a whole world of uses for waterproof bags that versions without a shoulder strap can't be used for.
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Some of these uses include

Surfing and SUP: Whether carving the wave or taking your SUP down an estuary, you now don't have to worry about burying your keys in the sand and wondering where you left them. Plus, if you are taking your SUP for a flat water cruise, you can take your phone, wallet and keys, hey, maybe even a cool beer or two with you. Put your gear in your waterproof bag, sling it over your shoulder and away you go.

Cycling: There's nothing worse than riding to or from work with your gear in a backpack, only for it to get soaked but a sudden down pour. Put it in your waterproof bag instead and you're laughing. You might get wet, but your gear won't.

Jet skiing: Sure, laps along the one stretch of water may be fun, but there's nothing quite like taking your jet ski further afield and doing a little exploring. With your bag over your shoulder, you can take the necessary gear with you on the jet ski for a nice little trip. It may just be a few extra clothes, but may be you'll get a little romantic and pack a picnic.

Caving: Yep, it's just what you think it is, exploring caves. Apparently caving can get quite wet and muddy - just the sort of place a waterproof bag would come in handy. But one that you need your hands to carry is no good, your hand are busy getting you through the caves. That's why cavers need a waterproof bag with a shoulder strap.

Kite surfing: Like surfers, or jet skiers, kite surfers will benefit from a waterproof dry bag with a shoulder strap as they'll be able to go with the wind for as long as they want, and still take their mobile phone with them in case the wind stops and they need a lift home!
There's no end to the things you can use a waterproof bag with shoulder straps for - you'll be surprised at the number of uses you'll find.

2. They can be secured more easily to things

When you only have the roll seal, there's not much that you can use to secure your waterproof bag to something like a boat etc. However, with a shoulder strap, you can use the shoulder strap to hang or tie the bag to a railing or other item. Plus, as the shoulder strap will require buckles on the waterproof bag to attach to, these can also be used to attach it to things. For example, you could loop a bike lock through the buckle and securely fasten it to something. Or you could hang it on a hook and still access the bag. And of course a shoulder strap means you can attach it to your body to it's always close by! Having a shoulder strap makes your waterproof bag so much more versatile. This means you will use it more and get much more value for money.