Cometary water group ions are the dominant population controlling the interaction of comets with the solar wind. The distribution function of this population has important effects on the overall structure and boundaries in the comet-solar wind interaction and on the plasma wave activity produced. In this paper we present new results on the behavior of the water group ions measured by Giotto at comets Halley and Grigg-Skjellerup. The pressures perpendicular and parallel to the magnetic field are presented and the implications explored. We show that during the Halley inbound pass, the firehose instability may excite the observed waves with low wavenumber. Also, we find that the energy density of energetic cometary ions is greater than the observed difference between the energy released from the ring-like distribution and the energy in the pickup-excited waves. This indicates that another energy source must be contributing to the acceleration, probably related to the deceleration of the solar wind in the comet frame. ¿ American Geophysical Union 1996