The safe way to bath a baby when you have a BPI. Even in a baby bath you can either hold the baby's head above water or wash her... NOT  both.

You can get special things to hold the baby in the water but frankly, lifting a baby in and out of the water is a killer with one arm, buckets are best! Because the baby is held upright I used to take the opportunity to feed her too, note the saucepan of icky baby goo next to me. Feeding a baby who can't sit up properly yet is not easy with one arm, more goes on than in. I was also able to carry the bucket around the house with me. NB the baby must be well soaped or it's not easy to get her out with one arm. 
This pic is of Catie, my eldest who is now 14, hence the hideous 80's boots and jeans I'm wearing...........
Check out these links for more info;  

Baby care experts and midwives, especially in Europe, recognise that this method of bathing babies up to the age of two years is very safe and beneficial to the child, and to the parent who uses one or both arms. The 'Tummy-Tub' that you can see on these sites has been developed from buckets that were used for many years by Dutch parents. 

Unfortunately, I have not found anywhere in the U.K. who sells the Tummy Tubs or anything like it..
If you do or are thinking about bathing a baby using a bucket in this way, please observe sensisible safety precautions. Never  leave a baby unattended in a bucket of water!

Here are a few more links to sites that may be useful. These are sites specifically for and by disabled parents. Some useful info on these; also some useful bits and pieces for coping with a baby or lively toddler single-handedly.
How to bath a baby one handed,  by Jenny B.
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