Oh, fun! It doesn't surprise me that something might be missing from the Scrabble dictionaries. Traditionally, the Official Scrabble Players Dictionary pulled from just "five in-print collegiate dictionaries, namely The Random House College Dictionary (1968), The American Heritage Dictionary of the English Language (1969), Webster's New World Dictionary (1970), Merriam-Webster's Collegiate Dictionary (1973) and Funk & Wagnalls (1973)" (quoting https://en.wikipedia.org/w/index.php?title=Official_Scrabble_Players_Dictionary&oldid=698206686).
So I looked up undine on an online version of the OED (subscription only, sadly). At the bottom of the entry, it has a "Draft additions 1993" section which has information about undinal--it references the 1891 Century Dictionary definition--which brings us right back to the Century definition here on this Wordnik page.
A recent edition of the TV programme Countdown announced undinal as a valid English word, being an adjective derived from undine, a water spirit. But it does not appear in my Oxford English dictionary, nor any similar Latin dictionaries (except as unda - a wave), nor the SCrabble dictionary or any other according to Wordnik (this page). Etymology apparently stems from the 16th or 17th Century Latin coinage.