Numerous underwater image enhancement schemes are being used for improving an image, which includes gray scale manipulation, filtering and Histogram Equalisation. Histogram Equalisation has become a popular technique because this method is simple and effective. Another significant technique in underwater image enhancement is Discrete cosine transform. Discrete cosine transform is a fast transform that has excellent compaction for highly correlated data. DCT gives good compromise between information package ability and computational complexity. In this paper underwater image enhancement is proposed based on discrete cosine transform and Unsharp Mask Filtering, which gives the significant results as compared to previous techniques.