I thought I would love floating around in the bath while I was in Labour, and I got in there and I hated it, couldnt relax and quite frankly I couldnt wait to get out ( I think I lasted about 20 mins) but I think the main thing to keep in mind while writing your birth plan is to put down anything you think might help, so if you think the bath might help then I would certainly add it into my plan. If you find something isnt working for you then you dont have to stick with it, you can give something else ago.
So when writing your plan add in as many things as you think might help, so that if one thing doesnt work you can go on and try something else.
In the end when I ws in Labour with DD I had a bean bag up on the bed, with the bed head tilted up and I just laid over the top of it for ages, and I think that was the most comfortable thing I found. Doesnt mean it is the only thing I will try this time, but it will definatley be on the list.