HumanCartoon said..
However you carry it:
PFD
Leash
Hydration
Some food (I carry a couple of energy gels and/or a bar product)
Phone in waterproof case (with a full charge)
And make sure you:
Have adequate fitness
Aren't alone (especially if inexperienced) or if going solo make sure someone knows where you're going from, where you expect to finish, and your ETA.
Dress for the conditions (especially in cold waters - better to be a bit too warm than to be cold)
Good tips there Stu..

I think paddling a yellow board adds even more safety..