Older models of snorkel vests often included CO2 inflators as well as oral inflation tubes. However, when used consistently in salt water, CO2 cartridges tend to corrode. As a result, snorkel vest manufacturers discontinued this feature and now use only oral inflation tubes to adjust the buoyancy.
If you’re concerned about swallowing water or experiencing an emergency in the water, it’s best to leave your vest inflated for the entire duration of your snorkeling adventure. Alternatively, you may purchase a PFD with a CO2 cylinder like the Hyde Wingman Inflatable Life Jacket at Amazon, but bear in mind that these types of flotation devices are not specifically designed for snorkeling.