Do they NEED them, most probably not, but I always put one on mine for two reasons. They camo in better and the vest themselves have some bouyancy. I Hunt big water and feel safer both from the Temp aspect but also if they were to get into trouble the little extra bouyancy might give me time to get out there and help.
If you can find the ones, I forgot which brand, with a ZIPPER on the back and a handle to help pull them in the boat on the back along with some extra floatation I would get it. I have seen VELCRO vests come off, kinda embarrasing to have to send the dog back after its vest

But the velcro can gget weeds seeds in it or just plain wear out.
Most importantly though, make sure the VEST FITS RIGHT, and don't be afraid to trim it some to make it fit better