In my opinion, the first thing that I would do is to throw an object that floats out directly to them. I would also find a row boat and row out to help them. If I cannot get them out I will dial 911 right away.
First and foremost, LAUGH. Second, ask them if they can swim (If they say yes to being able to swim, then tell them to swim to shore, BUT, if they aren't answering your question b-cuz they are too busy choking on water and flailing uncontrollably, then you should probably jump in and rescue them)
The first thing I would do to help them is stop laughing, next I'd put my camera/phone down. If I knew they couldn't swim I'd strip off excess clothes/shoes quick and dive in. it really depends who it is, for some I'd help them by holding their heads down