Electronic “waterproofness” is rated IPX? where a water resistance number goes in the slot where the “?”is (Most smartphones are rated IPX7 but that “7” is not adequate for extended immersion). (IP ratings - if there is a number where the 'X' is in “IPX7” that represents resistance to dust/dirt exposure and entry. An 'X' means it has no rating.

The number at the right end is the waterproofness or really is a water resistance rating. Even with an IPX8 rating, keep in mind that the standards testing is done in still, fresh water - not in water with any kind of movement or force acting against phones being tested. And the rating does not mean that there is any protection from chlorine or salt water. Better to use a case, period. The charging ports are particularly vulnerable. Even if your gadget continues to work, a charging port like a USB port that gets wet can have issues getting dry enough to work for recharging. And USB ports or sealed, stainless steel extruding charging connectors will corrode from regular contact with sea water. It is highly advisable to use a waterproof case with IPX7&IPX8 rated electronics (or if you don't mind risking your electronics or need to have it more readily accessible, at least put sealing tape over the charging ports/connectors).