Retired last year... 28 Years in the Coast Guard... Spent almost 16 years of that time at sea. I've witnessed a lot of terrible things when it comes to people and water, but with that being said... Being out on the water is usually a joyous experience. As mentioned many times already... A PFD is paramount, but a couple other things I would like to suggest:
Always let your better half (or significant other) know where your going and where your launching, and an approximate time you'll be back. That way if you do get into trouble, they'll know when to call for help and where to send it.
Also, don't keep your phone in your pocket while in the boat. If you fall into water with your cell, it won't be any good to you when you get back into the boat and should need to call for help.
And lastly... Take it slow and steady in congested areas, around other boats, and/or unfamiliar waters. Speed Kills!
Have a safe Coast Guard boating day