I have done a Swimrun and am training for another. I have used an older 1l camel back in a race and training. The sides are tight to the bag and the overall size is small. It did not take on much water (other than what was in the bag) and was fine to swim with.
Some friends did the race with full wetsuits and backpacks. They stopped to put on the wetsuits at every swim and swam with the mostly empty backpacks on. The backpacks were regular day packs/school bags. Do not do this! They were like sea anchors and were even hard to get out of the water at the shore.
I have a large safer swimmer float. Great device. Limited carrying capability. Remember, everything you add that is as dense a H2O is going to to reduce the flotation of what ever it is in or on. This will increase your drag in the water. Look into some of the filtration systems like lifestraw to reduce weight if in freshwater.
What ever you plan to do, make sure you have tested it out in a safe way and done some training with it. Also make sure you can start a fire if necessary. Hypothermia is a real bitch.