Vacuum sealing with a submerged ziplock vs home machine vs professional machine
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.