The good time is when the vessel is in the water, in commission, is fully operable, can be hauled for bottom survey, and the weather is fair, warm and lovely.
Marine surveyors have to do it both ways.
The winter survey is not quite the fiasco that some think it to be, but it does have limitations. These are only in addition to the limitations that, inherently, are in any marine survey no matter how ideal the prevailing conditions are and no matter how thorough the surveyor is. There are structural attachments and secondary bonds there that we would love to get a look at, but we do not have the freedom to tear apart a vessel just to see that all is ok. We can't see inside, behind and under tanks, or inside an engine or transmission. While we use implements like the plastic hammer and the moisture meter, mostly we rely on our perspective, knowledge and common sense to discover and identify invisible problems.