@Albert Great question!
To answer your question, yes, it is okay to use water to flush the salt and sweat out of your backpack. @Wanderer is also correct that you can use a non-detergent based cleaner (like Nikwax) to help get it even cleaner. If you have particularly stubborn spots you can also use a light brush (like an old toothbrush) or sponge to rub the soap in. Then rise it clean and let it drip dry.
Hope this helps, thanks!