Question: In snorkel equipment what do the sizes represent? 

Answer: Fins and Booties come in Men’s sizing with Women breakdowns. 
If you are speaking about Mask Sizes for say Small to Medium Faces, then these 
take into account facial features that may be an issue when it comes time to fit.  
Women usually fit in Small to medium masks where Men tend to be fine in either style. 

Question: What size for an average size male? 

Answer: Medium to Large

Question: What size for an average size female?

Answer: Small to medium

Question: What is a semi-dry snorkel? What are options?

Answer: Semi Dry comes with a Splash or Wave Guard to keep water out if a wave comes.  
A dry snorkel will stay dry inside when submerged.  For regular snorkeling most folks 
go with the dry one like the Oceanic Dry Snorkel.

Question: Do the inflatable snorkel vests adequately replace the larger/bulkier 
"life saver" type vests? Reason - traveling and looking to save luggage space.

Answer: Yes a snorkel vest will pack deflated and is specifically used for snorkeling 
where one may need the floatation but is mainly a precaution.  They are also nice to add 
a few breaths of air while resting on the surface.  They are not to take the place of a 
life vest so whomever uses a snorkel vest must be a good swimmer and comfortable in the water.