Rule No 1
If you are already in a harbor or at anchor, stay in the shelter and wait for the visibility to improve.
Rule No 2
If underway try to find an anchorage or harbor immediately and stay away from main shipping lanes. Quit often it's shallow enough on the side of such a lane to stay at anchor.
If you have to move, plot a fix in the chart and follow this 10 commandments for your own safety and the safety of your crew and vessel:
1. Put on Lifejackets
Make sure all hands put on lifejackets. Fasten all straps and snaps. Attach flashlights to each lifejacket. Stuff a wool or microfiber watch cap in your pocket. If you go into the water, this will help slow down the onset of hypothermia.
2. Switch on your marine navigation lights
The navigation rules require all vessels to show proper lights during darkness or reduced visibility -- day or night. Sailboats show red and green side lights and a white stern light.
If you have your engine on, turn on your steaming light. If available use a stroboscopic light additionaly (strobo lights are not IRPCS but help a lot to be seen...)
3. Warn others of your position
Use a manual fog horn or air canister horn to warn other vessels of your position and actions. Sound fog signals in or near areas of low visibility. If you're sailing near a fog bank you need to sound fog signals to warn vessels inside the bank of your position.
4. Maintain a lookout by "all means"
Send one crew forward to the bow away from the noise of the engine. Hearing becomes the most important of your senses in fog or haze. If you hear or sight another vessel, slow down or stop. Figure out what the other vessel is doing before you proceed. If you have installed radar, the navigation rules require that you use it along with visual scanning.
5. Hoist radar reflectors
The best designs have three round panels that bisect one another at 90 degree angles. Hoist the reflector so that the top panels form a "V". This presents the best reflective surface when heeled over. Hoist the reflectors high up on your backstay and shrouds.
6. Use a high-beam spotlight
Keep a powerful, one million candlepower, handheld spotlight in the cockpit. If a ship approaches, shine the light onto your sails.
7. Rig jacklines fore and aft
Jacklines provide an attachment point when moving between the cockpit and the bow. Run two lines or long lengths of flat webbing on each side of the boat. Attach the lines or webbing to the bow and stern cleats. Remove as much slack as possible so that they don't become tripping hazards for your sailing crew.
8. Wear safety harnesses
Wear a safety harness. Clip on to the windward side jackline. Strap a sharp knife in a sheath to your belt. If your safety harness gets tangled, you want to be able to cut yourself free in an instant.
9. Stop, listen, and look
Heave to often, stop the boat and listen. If under power, stop the engine and listen for several minutes. To find a sound equipped navigation buoy (bell, whistle, gong), make a tight circle to create a wake. Then, stop the boat and listen.
10. Get your boat anchors ready
Stand by on your bow anchor and have your second anchor ready in the cockpit with a long line. Make sure you can get your anchor over the side in a few seconds. If unsure of your position, anchor until you sort things out. If underway and you hear breakers ahead, lower the anchor right away.