IP67 means protection against temporary immersion, conventionally one metre for thirty minutes. IP68 means continuous immersion at a depth the manufacturer states. The second digit is not a grade of the first; it describes a different test and a different expectation.

A light bar on a bumper that occasionally fords a river is an IP67 application. A hull light that is permanently below the waterline is an IP68 application, and no amount of IP67 testing predicts how that unit behaves after a season submerged.

The rating also says nothing about salt. Immersion testing is done in fresh water, so a marine unit needs marine grade housing material and sealed cable entry on top of its IP figure. Salt water attacks the fixing and the gland long before it attacks the lens.

Ask what depth the IP68 figure was tested at. IP68 without a stated depth is an incomplete claim, and for underwater lighting the depth is the whole point of the rating.