Found this about IP ratings for dust/particles and water:

IP-Ratings

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
6 Tight Dust-tight.

Second Digit = Water

1st Digit Size Angle of Incidence Protected against...
0 None Any No Protection
1 Drops Any Occasional water drops
2 Drips 15º Water drops
3 Spray 60º Water spray
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"

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.

Marine Applications

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
Batteries 3 4 IP34
Bilge Pump 3 7 IP37
Bus Bars 3 4 IP34
Chartplotter 3 7 IP37
Decks and Hull 6 7 IP67
Electronics, Critical 5 7 IP57
Electronics, Non-Critical 4 6 IP46
Electronics, Discretionary 4 5 IP44
EPIRB 6 7 IP67
Food 6 7 IP67
Hatches 5 6 IP56
Portlights 5 7 IP57
Roller Furler 4 7 IP47
Sails 0 4 IP04
Water Pump 3 6 IP36
Water, Distilled 6 7 IP67
Water, Drinking 5 7 IP57

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.