A numerical analysis on deck wetness is carried out for a large caisson directly wet-towed by tugs in irregular waves. A constant panel method is used for linear analysis in frequency domain and a statistical postprocessing for the deck wetness is presented. Hydrodynamic coefficients obtained from the frequency domain computation are imported for time domain analysis which enables complete modeling for towing equipment, environment, etc. Both frequency and time domain computations over two sea states are performed and comparison is made. In the time domain analysis, towing systems of various arrangements of tugs are investigated from shortterm prediction for the largest deck wetness and the number of occurrences of deck wetness.