The Pacific Island region is an ideal setting for case studies on the invasion history of macroalgae in tropical coastal waters. Many textbook examples of algal invasions in this region demonstrate their significant ecological impacts on native reef communities. In light of increased maritime traffic as a potential anthropogenic dispersal mechanism for nonindigenous marine algae between tropical North Pacific Islands, a risk assessment was conducted to (1) quantify floristic disparities between marine ecoregions based on three subsets of species inventories, (2) evaluate the environmental and spatial distribution characteristics of island floras that are conducive to new introductions, and (3) establish a baseline with which future risk assessments based on a molecularly-assisted alpha taxonomy concept may be compared. The results of the environmental and floristic analyses showed that the “potentially invasive species” (i.e., species belonging to genera with known invasive representatives) constitute the best taxonomic subset for future risk assessments of marine macroalgae as this group (1) consists of an adequate—yet manageable—number of species, (2) is characterized by environmental and distribution parameters similar to that of the entire flora, (3) alleviates the deficit of non-indigenous species detection in tropical waters due to a historical bias toward temperate waters, and (4) closely mimics proportional differences in species composition between entire floras.