Preparation
You want to be ‘seen’ in fog so invest in a good radar reflector and permanently fit it as high as possible. A radar reflector is one of the few SOLAS V requirements for small craft. Which one you choose will depend on the depth of your pocket, but be wary of going for cheap and cheerful. You can spend hundreds of pounds on a radar target enhancer, or you can buy a passive reflector for much less. If you go for a traditional tetrahedral reflector just make sure you hoist it in the ‘raincatching’ mode, not point up.
AIS does not yet feature in the Colregs but it can be an invaluable anti-collision aid. Even receive-only sets will make life much safer. However, AIS is not a radar (whatever some manufacturers may claim), and many small craft are not fitted with it; even larger vessels may not have it set up correctly or even turned on.
Like AIS, the use of VHF is not mentioned in the Colregs, and you should be very wary, even in clear weather, of causing unnecessary confusion by calling another ship to negotiate actions to avoid a close-quarters situation, unless you have both positively identified each other. This applies in poor visibility when a third vessel may be tempted to take avoiding action based on an overheard conversation. That said, particularly in busy areas, a listening watch will help you to build up a general picture of shipping movements around you. So, have your set tuned to the relevant channel(s): Ch 16 when offshore as well as the local harbour or VTS channels when appropriate. If you feel you must check your radio before sailing, try to do so on a working channel and avoid cluttering up Ch 16.
The advantage of radar over AIS is that, when used properly, it will show all other vessels, and not just those fitted with corresponding equipment. With either AIS or radar, however, the information you are given still needs interpretation. You should be able to read it and ascertain another vessel’s closest point of approach (CPA), it's time to CPA (TCPA), and where it will be in relation to your vessel at that time.
If you have radar, it is a requirement of Rule 19 that you make proper use of it in reduced visibility. Therefore, tune it for best reception and know how to get the most out of it. Unless you are already an experienced operator, book yourself onto a course then practise in clear visibility. You won’t regret it when your safety depends on it.