My compliments on your question and for showing a maturity level not often seen these days. Even though snorkeling is more enjoyable when it is a shared activity, there are situations where you may be the only one that wants to snorkel. If this is the case, it is a good idea to take some additional precautions, one of which is to where a highly visible snorkeling vest.
This will make you easier to spot for anyone looking from shore or a boat. The vest is usually worn deflated or partially inflated. if you get tired out you can inflate it orally to allow you to rest, relax and recover.
Having a whistle with you is also a wise idea should you encounter a problem and need assistance. Make sure the whistle isn’t made of shiny metal as we humans tend to use shiny things as fishing lures. This is also a good reason not to wear jewelry while swimming or snorkeling in the ocean.
Having someone you know on the beach is always advisable but letting someone know where you will be snorkeling, how long you plan to be out there and when you expect to be back should always be a part of your snorkel plan.