Activities of the Monowai Volcanic centre have been observed as seismic waves recorded at Rarotonga station (RAR), as well as hydroacoustic waves recorded at Ascension Island (H10). Beamforming has been performed on the hydroacoustic data to verify the source and study the effect of performing calculations with a best beam. The lag time that is calculated by cross correlating seismic and hydroacoustic data. With the known source-receiver distance, the sound speed and hence the temperature can be calculated. Finally, the lag time is studied as a function of time in order to observe seasonal changes. In general, travel times between the source and hydrostation H10 do increase with decreasing temperature. In the Southern Hemisphere winter, however, an unexpected decrease of lag time is observed. This might be explained by a different propagation path due to the formation of sea ice.