Among biomass resources for bioenergy, marine macroalgae such as Ulva lactuca is highly productive, requires no land or freshwater, and remains underexplored compared to terrestrial biomass. Its high moisture content makes hydrothermal processing or subcritical and supercritical water gasification (SbWG and SCWG) well suited for conversion to bioenergy, eliminating the need for drying. Chemical additives such as formic acid can further enhance the process. This study investigated the effect of formic acid on SbWG and SCWG of Ulva lactuca for hydrogen and hydrochar production. Experiments were conducted in a 500 mL high-pressure reactor at 300–400 °C for 30–90 minutes, using feedstock concentrations of 1 wt% and 5 wt%, with and without 1 wt% formic acid. Gas, solid, and liquid products were collected and analyzed (proximate, ultimate, and HHV). Results showed that formic acid significantly enhanced hydrogen production, with a maximum yield of 39% v/v at 350 °C for 90 min. However, hydrogen yield declined at 5 wt% loading, likely due to inefficient mixing. Hydrochar quality deteriorated with formic acid, showing lower carbon content, higher oxygen, and greater moisture, producing a sticky, tar-like material. The Van Krevelen analysis confirmed that formic acid promotes the formation of gaseous products over solid carbon retention, making it more effective for enhancing gas yields rather than producing solid fuel.