tori32: "To do this holding a baby you would have to hold under the back or front and is not safe."
As long as you know how to do it it's perfectly safe. Our pool's water babies sessions are as much for parents to learn how to do these things as they are for babies.
tori32: "Baby can tip forward or backwards into water, taking in huge amounts of water for its lung capacity."
Not if you start them early enough (sub 6 months-ish) they won't. At that age they have a dive reflex which means they just won't breathe in water. Yes, it was bloody scary the first time I let go a 3 month old at arms length and watched him paddle underwater - but he didn't inhale a drop. Later on this reflex needs to be relearned, which is why starting 'em young is a good idea. They do hate getting water up their noses, though, so it's a good idea to do these "swims" starting from a horizontal position so the head can't go far enough for the nostrils to slope "down", if you see what I mean.
tori32: "Also, if you have more than 1 child to supervise and you have to hold a baby, how would you help the other child if he or she got into difficulty?"
Very few pools will let you supervise a baby and a child at the same time: I'm on the board of my local (charity run) pool and we only allow one under-4 per adult. It's the same at the council pools here as well. There's a good discussion of this at the Institute of Sport and Recreation Management website.
I'm not trying to be rude when I say you sound a bit scared about this. Which is natural, and I found taking The Boy swimming very scary at first. A good parent-a-baby class is really useful, partly because you learn lots of useful stuff (I did) and partly because you see lots of other worried-looking parents and realise that you're not alone (I did that too!).