The design of large catamaran sailing yachts requires careful balancing of aesthetics, comfort, and performance, with the wet deck geometry playing a decisive role in resistance, motions, seakeeping, and slamming behavior. In this work, a method is proposed to enhance the estimation of wet deck geometry for large catamaran sailing yachts in an early design stage. While the operability of such catamarans is primarily driven by passenger comfort, the influence of wet deck geometry on motions, seakeeping, and slamming was identified as the main factor limiting vessel operability. Four key (non-dimensional) parameters, including wet deck height (h/L), hull separation (s/L), demihull beam-to-draft ratio (b/T ), and the starting position of the wet deck relative to the bow (Lswd/L), were systematically varied, resulting in 295 design configurations. Vessel performance was evaluated using a comfort operability index (cOI), which quantified the percentage of time a vessel could operate within the predefined comfort limits. The cOI was derived from seakeeping criteria, including effective gravity angle, motion sickness incidence, vertical and lateral accelerations, and slamming probability. Response amplitude operators (RAOs) at the center of gravity of each design were obtained using Maxsurf Motions, supplemented by an in‑house Python post‑processing tool to evaluate the seakeeping criteria across multiple onboard locations. An engineering correction was applied to heave motions in beam seas. The analysis covered three vessel lengths (50, 60, and 70 m), three headings (beam, bow‑quartering, and head seas), and representative sea states for the Mediterranean (Jonswap spectrum) and Atlantic (Bretschneider spectrum). Results showed that increasing vessel length consistently improved cOI, particularly in head and bow‑ quartering seas. Among the wet deck parameters, s/L strongly influenced lateral acceleration and effective gravity angle in beam seas, while b/T and Lswd/L significantly affected motion sickness incidence and vertical acceleration in head and bow‑quartering seas. Higher b/T ratios generally improved comfort, though the impact depended on vessel length, heading, and sea state. Lswd/L exhibited an initially positive but ultimately negative influence at higher ratios, while h/L showed an impact only in beam seas under Mediterranean conditions. Differences between the Jonswap and Bretschneider spectra were modest, with the Atlantic spectrum yielding slightly higher cOI values and reducing the limiting role of motion sickness incidence. Across all conditions, forward onboard locations emerged as the most critical for limiting comfort. By quantifying the impact of wet deck geometry parameters on operability, this study established a methodology that provided deeper insight into the influence of wet deck parameters during the early design stage. While the present analysis excluded sail forces and steady heeling angles, these aspects could be integrated into the framework of this study in future research.