I use snorkels in limited way. For the 'true sinker', I have them use the snorkel in drill and whole stroke until they've discovered how to expand their lung capacity and keep enough air to remain buoyant.
Second, for the novice or intermediate swimmer that struggle with the breath, changing body position to get air - I have put this on them to establish stroke symmetry and stability and then integrate breath. But always short sets, i.e. 50y with snorkel, 50y without.
Lastly, establish good head position. If head and spine are in correct alignment, snorkel will be 2"-4" above the surface (both in drill and whole stroke). If head goes out of alignment or stroke asymmetry causes swimmer to bob up and down - it's down periscope, swimmer is sucking water. This is great feedback for the swimmer that doesn't have a coach on deck telling them their head is out of position and/or bobbing up and down during whole stroke.