Dozey - I had a waterbirth with DS2 and tore - but along the lines of the tear I had from DS1 and was only in the water for 15 minutes before DS2 came out so the water didnt have a chance to stretch/supple the old scar tissue if you see what I mean. Only needed a couple of stitches and was fine.
Are posting all of these threads really helping you get ready for childbirth or freaking you out more? I'd say, relax, enjoy the remainder of your pregnancy, go with the flow in birth, listen to your body and let your midwife know your wishes and focus on your breathing and you'll be fine.