Vacuum sealing with a submerged ziplock vs home machine vs professional machine Typically I'll seal food for storage or sous-vide in a Ziplock bag, submerging it in water whilst closing the final inch to remove air. 

How close to a machine seal is this, and is it worth worrying about both for storage and sous-vide?

Can salts be dissolved in the water or some other process to both increase the pressure of sealing and maintain a vat of water for longer without nasties growing in it?

With a ziplock under water, I feel goods without concavities seal perfectly and definitely don't crush the food. Plus of course liquids easily seal. Sealing soup bones however will have the plastic somewhat conform to the concave contours, but bridges are seen. Does the plastic somewhat stretch or bunch perfectly to the inner contours with a pro sealer?
For storage, the biggest issue is whether you have a high-quality thick plastic bag that will prevent oxygen incursion. Have a few small bubbles inside isn't a big deal, whether you use the water method or a vacuum. But cheaper/thinner bags will allow oxygen in over time, and your bag will start to separate from the food, and you'll start to see ice crystals.

For sous-vide, I'm not an expert, but I think the biggest issue is that air insulates the food and prevents it from attaining temperature quickly and evenly. I think people who do home low-temp cooking recommending having a sauce or fat in the bag along with your protein to reduce the issues with bubbles. I do know that professional chefs will use the ziploc-in-water technique at home, with good results.

I don't think adding anything to water will affect the density/pressure enough to matter. 

And yes, professional vac machines don't have this problem.