This study examined the mental workload (MWL) of cockpit work on high-speed workboats’ (HSWs), as the crews of these boats are exposed to harmfully high and low MWL, which are both detrimental to safety and performance. MWL was measured during observed, real-life voyages using an experimental subjective Likert scale that assessed cognitive reserve capacity that could then be compared to retrospective NASA TLX data. The crews were interviewed about the factors that affected their MWL during voyages and the need to improve MWL management in the sociotechnical system of the HSWs. The MWL measures were sensitive to cognitive load variations and suitable for measuring MWL in the maritime field. The crews' MWL, but on the other hand cognitive reserves were repeatedly reported as low. Crew competencies in particular were seen to improve workload management, which was especially weakened by insufficient skills, the technology used, its usability, and the cockpit layout. The interviewees made proposals for how to enhance MWL management. The results contributed to improving MWL management and the mastery of human factors in HSWs and small-craft operations.