Also, if you can afford it, a marine VHF radio. Obviously a whistle or airhorn is a good idea. Here is some quick stuff on this. I think by next year we will have worked out a good set of guidelines on this. So this is just to tide you over.
Swimmers vary in their ability to hear and carry on a conversation.
Some have seemed totally deaf to anything but a mine going off. Others catch your words quickly. Try everything you can to communicate. But if the swimmer is going off course or into danger, position your kayak in front of them so that they either hit it or see it and stop. We are making certain in the NYC swims that the race announcer makes clear to the swimmers that if a kayak is in their path it is for a reason, i.e. to help them stay on course or away from a danger.