Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
1. summersus (subm-), a, um, Part. of summergo.
2. summersus (subm-), ūs, m. [summergo], a plunging under water, sinking (post-class.), Tert. Anim. 32 med.