IPA: /ˈʌndərˌsiːz/
KK: /ˈʌndərˌsiːz/
In a position or state that is beneath the surface of the sea or ocean.
The treasure was hidden underseas, waiting to be discovered.
Underseas → It is formed from "under-" (meaning beneath) and "seas" (the plural of sea, from Old English "sǣ", meaning large body of saltwater). The word "underseas" refers to something that is located beneath the sea or ocean.
Think of being 'beneath' ('under-') the 'large body of saltwater' ('seas') — that's what underseas means.
No commonly confused words.