The search for life beyond the solar system necessitates understanding the atmospheres of rocky exoplanets that reside in the habitable zone. An important component of building and maintaining an atmosphere is magmatic outgassing. The TRAPPIST-1 system contains seven terrestrial exoplanets and may provide an opportunity to observe and compare the outgassing products of terrestrial exoplanets. Mass-radius observations suggest that the habitable zone TRAPPIST-1 planets are potentially volatile rich “waterworlds” with substantial inventories of surface water. Here, we explore the relationship between ocean depth, temperature, and outgassed species to make predictions for plausible atmospheric compositions testable with the James Webb Space Telescope. We focus on the potential for high pressure methane outgassing as a potential false positive for biogenic methane. We calculate fluxes of methane expected from volcanic activity to estimate an abundance threshold for distinguishing biogenic methane from non-biogenic methane. These calculations expand understanding of methane outgassing on waterworlds and help provide testable predictions for future spectroscopic observations using James Webb Space Telescope in the search for biogenic methane on TRAPPIST-1 and elsewhere.