Nicola,
This is a great question and one that I am also considering on our next trip, since this will be our first time taking our youngest to the water parks. My older kids are thrill-seeking and love the water, whereas my youngest is still a bit young and will be safest in the kiddie areas.
I would say the best plan of action would be to get to the water park early and stake your claim on some chairs near the kiddie areas such as Tikes Peak at
Blizzard Beach or Ketchakiddee Creek at
Typhoon Lagoon. That way, you are closest to the area that you will most likely not veer to far from rather than basing yourself near one waterslide, since your son will likely be moving all around the park from one ride to the next. Even if your daughter decides to go somewhere different, you can have an area already chosen as a meeting place or home base! Choose a time if you want to do a kid swap so the other parent can enjoy time with the other child. Then fun is had by all and you don't spend half your time searching for your party!
I would also suggest taking this philosophy of a home base or meeting place to the theme parks as well. We always designate a meeting place if we ever get separated. Keep in mind that you don't want to choose someplace that is spread out and could be hard to locate your party even if you are where you think you are supposed to be. For example, choose a place like the entrance to Casey's Corner at Magic Kingdom rather than just picking Cinderella's Castle as your meeting spot since you could be in front of, on the side of, or behind the castle.
I hope this helped. Have a splashing good time and be sure to apply sunblock regularly and stay hydrated!
~Allison L