Read Aadil's reply:
Yes, there are many other poses that you could teach in water. Though I would avoid Headstand and Shoulder Stand (!), students can easily do Uttanasana with their hands on the edge of the swimming pool to stretch their hamstrings. Other poses include Parsvottanasana, again with hands on the edge, and again for hamstrings. Students can do Gomukhasana and Garudasana to loosen up their shoulders. They can easily perform Warrior I (Virabhadrasana I) and Warrior II (Virabhadrasana II). A pose that I use regularly in the swimming pool is Utthita Hasta Padangusthasana, with the raised leg on the edge of the swimming pool. However, since you are working with elderly people, this pose will have to be modified. Have them place their raised leg on a step or another raised surface so they don't have to raise their leg too high.
After understanding the basic principles of yoga postures, you will be able, with a little creativity, to modify other poses, such as upper chest backbends and twists supported by the wall of the pool. I recommend a deeper study of the refined actions in postures so that you are able to modify them safely.
Recognized as one of the world's top yoga teachers, Aadil Palkhivala began studying yoga at the age of seven with B.K.S. Iyengar and was introduced to Sri Aurobindo's yoga three years later. He received the Advanced Yoga Teacher's Certificate at the age of 22 and is the founder-director of internationally-renowned Yoga Centers in Bellevue, Washington. Aadil is also a federally certified naturopath, a certified <a href="/health/ayurveda">Ayurvedic health science practitioner, a clinical hypnotherapist, a certified shiatsu and Swedish bodywork therapist, a lawyer, and an internationally sponsored public speaker on the mind-body-energy connection.