The term Sink Powered Underwater Acoustic Sensor Networks (SPUASN) refers to a special configuration within the recent Remotely Powered Underwater Acoustic Sensor Networks (RPUASN) paradigm, where the data sink supplies energy to battery-free sensors that constitute the network. In this paper, we introduce the Cross Layer Power Adaptive CSMA/CA (X-PACCA) protocol for SPUASN. Fully free from energy constraints of traditional protocols, X-PACCA integrates MAC, network, and transport layer functionalities. Packet relaying/routing is based on CSMA/CA backoff window size adjustment according to harvested power levels at nodes. End-to-end reliability is enhanced via acknowledgments sent by the sink. Congestion is avoided via prevention of redundant packet forwarding. Neither global network information nor synchronization is required at nodes. It is shown that, with appropriate selection of protocol parameters, X-PACCA achieves low end-to-end latency and high packet delivery performance.