Underwater Acoustic Sensor Networks (UWASNs) enable ocean monitoring but face severe acoustic channel challenges. This paper proposes integrating Long Range (LoRa) one of Chirp Spread Spectrum (CSS) techniques with Non-Orthogonal Multiple Access (NOMA) for UWASNs. The framework combines CSS modulation's interference resilience with power-domain multiplexing to enhance spectral efficiency in challenging underwater channels. We present uplink and downlink system models incorporating realistic path loss, ambient noise, multipath, and Doppler effects. MATLAB simulations demonstrate that LoRa-NOMA achieves aggregate spectral efficiency of 0.458 bits/s/Hz compared to 0.153 bits/s/Hz for equivalent orthogonal multiple access. Also attains enhanced improvement in total system throughput while maintaining acceptable user fairness index. LoRa with lower spreading factor (SF)=8 accomplishes reliable communication with bit error rate (BER) < 10⁻³ at signal to noise ratio (SNR) as low as -15 dB. Successive interference cancellation (SIC) feasibility is validated, with uplink showing superior performance through centralized interference management. The best performance user achieves BER approximately 10⁻⁴ at 15dB SNR. These results establish LoRa-NOMA as a promising solution for energy-efficient underwater networks.