My best advice is practice falling, and don't be afraid to get wet (weather and temps permitting - hopefully you're dressed appropriately for temperatures and conditions).
Recently a friend of mine had a scary experience which has served as my motivation to write this blog.
As a newbie SUP'er you are going to fall and it's ok. It's part of the learning process. Getting back on the board from in the water will probably be more difficult than you would think it would be the first few times it happens. This is why you need to practice falling. Stay close to the shore, stay in an area where you can stand up if you have to and practice getting on and off the board while you are in the water. As you start to master the skills needed to get back on the board then move further out into the water, and practice some more!
I am someone who wears a PFD no matter what time of the year it is. That 15 pounds of float (or X amount depending on your particular PFD) is a nice boost, it will help push your body up and ease the burden of having to push/pull your entire body back up onto the board. It can also be a bit of a catch 22, while it is helpful in pushing you up out of the water, it can create a bit of drag as you pull your body onto/across the board. Don't let this intimidate you. You can overcome this, it is NOT a reason for why not to wear a PFD. This is why you practice and do so until you are proficient and confident that if you are in 30 feet of water and smack in the middle of a huge lake that you can get back on your board after a fall.
Boards with handles or something you can grip to help pull yourself back up are ideal. If you can grab a hand hold do it. Otherwise you are going to want to maneuver towards the back of the board, push yourself up while simultaneously kicking and pulling the board into you. Once you are high enough out of the water now you have to get your legs up onto the board. If you have the upper body strength you might be able to push yourself up high enough to get a knee on the board. If not you'll want to be getting as much of your upper body on the board as possible and swinging your legs up and over onto the board.
Sounds easy right!? Master this in a lake before you move on to paddling rivers. Especially fast moving rivers with rocks, rapids and other hazards.
If you find yourself needing some adrenaline just imagine that you are about to get eaten by a gator! I experienced this for real while being a new paddler, I did some of my initial paddling on a blackwater river in South Carolina, with gators, cypress tress and the whole 9 yards. Being that the water was black you were unable to see what was underneath you. Super creepy feeling. As a new tipsy paddler I inevitably fell multiple times. Each time felt like a scene straight out of a bad horror movie. I was so terrified there could be a gator about to chew my legs off that pulling myself up with great oomph came easily. After all, chances are that no matter where you are you don't know what could be lurking beneath!
One last recommendation, especially if you are a new SUP'er. Don't wear an auto inflatable PFD. It will probably scare you half to death when it inflates, buying new cartridges will be an added expense, and once it is deflated it's useless. You might be tempted to want to deflate it so that it isn't in your way while you are trying to pull yourself back up onto your board. Don't do that. If you end up not being able to get on the board or falling a second time your PFD will now be useless. This is why wearing a traditional PFD is a better option.