As an example, you need to be able to read a chart like this one:
http://www.channelswimming.com/Doverst.gif
Further, you need to have an understanding of the effects of the wind and tides upon how you steer. (You can't just head in the direction you want to go; you need to do some vector calculations to factor in the powers of nature). You also need to understand radio procedures so that, for example, you know the difference between 'Mayday', 'Mayday Relay' 'Pan Pan' and 'Pan Pan Medico', together with knowing when to call into harbour control stations and when to speak to the coastguard authorities (together, of course, with knowing how to determine the correct frequencies for each type of call).