Found this about IP ratings for dust/particles and water:
The image in non-image format (there's no original content here):
First Digit = Sand and Particulates
|1st Digit||Object Size||Protection against|
|0||None||No protection against contact and ingress of objects|
|1||50 mm||Large surface of the boxy, such as the back of a hand. Solid objects greater than 50mm (about 2 inches) in size.|
|2||12.5 mm||Finger or similar object. Solid objects greater than 12.5mm (about 0.5 inches) in size.|
|3||2.5 mm||Tools or thick wire. Solid objects greater than 2.5mm (a little over 1/16 of an inch) in size|
|4||1.0 mm||Wires, screws, etc. Solid objects greater than 1mm in size.|
|5||Dust||Dust protected, but not dust tight|
Second Digit = Water
|1st Digit||Size||Angle of Incidence||Protected against...|
|1||Drops||Any||Occasional water drops|
|4||Spashes||Any||Water splashing from any angle|
|5||Jets||Any||Water jets from any angle|
|6||Strong Jets||Any||Powerful water jets and heavy seas|
|7||Submersion||Any||Temporary submersion for 30 minutes at 1 meter (about 3.3 feet) depth.|
|8||Custom||Custom||Customer specified. Not rigidly defined.|
|9K||High Pressure||Any||Higher pressure (eg Steam cleaning)|
the X digit
The X is there as a placeholder in case either the 1st or 2nd digit has no rating or isn't being considered for this particular rating.
I would assume that an X would be less than a 0 rating.
Protected, in all cases, isn't the same as "proof" or "fast". So while the design may provide water and object protection to the point of being waterproof, it may not be in all cases or for the life of the product.
Protection may also mean that the design accounts for and will recover from dust and water intrusion... or that water may intrude but the device will still function.
"Proof" or "Tight" as in water-proof and dust-tight is what you want if the item is to go through hell and back.
This means that for different areas of the boat, things should be rated for different things. Putting it all together, let's list out some parts on a boat to see what we would need...
|Area/Part||IP Dust Rating||IP Water Rating||IP Rating|
|Decks and Hull||6||7||IP67|
From this napkin accounting, it appears that for the most part, the minimum dust rating should be 3 while water rating should be 4.
Critical devices will need a water fastness rating of 7; though in some instances, a 6 will do.