Recently, underwater communication research has been interested about ocean environment monitoring, disaster surveilance, military purpose surveilance. However, underwater communication has been challenging because of limited bandwidth, multipath, delay spread and doppler spread. In this paper, For underwater channel analysis using the real sea experiment data used OFDM communication systems. In underwater environment, OFDM systems is very robust characteristic about ISI. but for OFDM system, channel estimation must be made for accurate data detection. channel estimation schemes is linear interpolation and DFT-based channel estimation schemes. estimated channel impulse response use for search a coherence bandwidth and coherence time. In this paper, I suggest valid signal threshold on the underwater channel impulse response at Korea West Coast sea shallow water. As a result of data analysis, ?8dB of valid signal threshhold is reasonable in Korea West Coast Sea shallow water.