Anyone who has taken a call on a boat deck or gone for a run in the rain knows that not all "waterproof" earbuds are equal. The number that actually matters is the IP (Ingress Protection) rating, a two-digit code that separately measures resistance to solids (dust) and liquids.
The second digit is the one to watch for earbuds. IPX4 means splash resistant from any direction - fine for light rain or sweat, but not something you should submerge. IPX7 means the earbuds can survive full immersion in up to a meter of water for about 30 minutes, which is a real margin of safety if they get knocked off a boat or dropped in a sink. A smaller but growing number of earbuds now carry IPX8, rated for continuous submersion deeper than a meter.
A few things the rating does not cover: it says nothing about resistance to saltwater corrosion over time, nothing about the charging case (which is very often NOT rated the same as the earbuds themselves), and nothing about whether the microphone still works clearly underwater or in high wind - those are separate engineering problems from simply keeping water out.
Charging case durability is worth checking specifically, since battery contacts are usually the first thing to corrode if a case gets wet repeatedly. CairoVolt's wireless earbuds guide breaks down IP ratings model by model rather than relying on marketing copy alone.