We present techniques to extend the sensing range in unrepeatered submarine cable systems by utilizing Enhanced-Scattering Fibre (ESF), large-area ultra-low-loss (ULL) fibre, and a digital Distributed Acoustic Sensing (DAS) interrogator. A DAS sensing range of up to 200.6 km has been achieved using 156km SCUBA125 fibre, followed by three segments of ESF. Additionally, we demonstrate long-range sensing capabilities and high-capacity data transmission over a 270.6 km unrepeatered submarine system, where DAS and 400G DWDM data transmission coexist. The impact of Distributed Raman Amplification (DRA) on sensing performance, and crosstalk between DAS and 400G DWDM channels in coexistence of DAS and unrepeatered transmission system are studied. Finally, we briefly discuss the potential application scenarios for monitoring undersea cables using ESFs.