I tried a water birth too, but although it felt good I had to get out in the end as I was too tired to stay up on my knees!
pina, I felt the same as you about writing a birth plan, but in hindsight I think that it was a good idea for me. I had a good labour - no horror stories here - but there are certain things I think could have been better if I had known what I was going to be like in labour.
As it turned out, in labour I go completely into myself - I couldn't / didn't want to communicate at all, although I could hear what everyone was saying. So I was relying on my birth plan and my husband to make sure the midwife knew what I wanted, which was important when she tried to give me a drug I didn't want. Because dh knew my preferences, he was able to speak for me when I literally couldn't.
My birth plan was very loose, ie I said that I would go with whatever was best for the baby, but my ideal was xyz. It just meant that dh had something to fall back on when asked to make a decision.
And second time around, a birth plan will allow me to write down how I was in labour before, and what I learned from that. Ie that I need to be reminded of certain basics, and that I can still hear what is being said (so please talk to me instead of about me!).
I feel that if all goes well and I get to the point of labour, I will be calmer knowing I have a birth plan because it'll be like I have a voice even if I can't speak again.