Know your skills and limitations. Thinking you are a better sailor than you are could be life-threatening, so under- -- rather than over- -- estimate your abilities. The same goes for the crew, if you have one.
Assess your vessel and determine how sea worthy she is. Like the skills evaluation, you are better to err on the side of caution. If the vessel needs any work, get it done before you head out to sea. If it is a long voyage, get an expert sea mechanic to give the vessel an over-all inspection, just to be sure.
Stock your vessel with the supplies you will need on your voyage. Calculate the number of people times the number of meals and then add some emergency supplies in case you encounter a storm and have to stay out longer than intended.
Make sure you take enough water for the voyage. While you can get by with eating only a little food every day, water is essential. If your sea voyage is in a tropical region, this is particularly important, as dehydration kills.
Check the weather forecast and get regular updates. Storms at sea can come up quickly and you don't want to be caught unaware. The same goes for knowing the tidal predictions and how they will coincide with your voyage.
Be aware of navigational dangers on your proposed voyage and be sure you can handle them. Keep your pilot book or almanac current and have an up-to-date chart.
Have a contingency plan. If something untoward happens -- like being injured -- make sure you can get help or get yourself back to land. Don't totally rely on your GPS; ensure that you have the skills to navigate if it doesn't work.
Make sure someone responsible on land knows your intended route, how long you expect to be gone and when you will be back. If possible, check in with the person at regular intervals.
