The PAN word for water was *daNum (Blust, ACD), *D2aNum (Tsuchida 1976), *daɫum (Wolff 2010). Barring special circumstances, this word should remain unchanged into Eastern Walu-Siwaish (EWS). Among EWS languages, Basay (one dialect) lanom ‘water’, Trobiawan zanum ‘water’, Kavalan zanum (in qi-zanum ‘to drink’), Amis radom ‘carry water from the water source’, Paiwan zaɫum ‘water’, PMP *danum ‘water’ show the expected outcomes.
Equally widespread in EWS languages are forms reflecting PEWS *nanum: Basay nanom ‘water’, Kavalan nanum, m-nanum ‘to drink’, Sakizaya nanum ‘water’, mi-nanum ‘to drink’, Amis nanom ‘water’, mi-nanom ‘to drink water’, Puyuma nanum ‘water’ (male, religious). Papora mananu ‘to drink’ (Tsuchida 1982) may belong here too—this would raise *nanum ‘water’ to Proto-Walu-Siwaish— but loss of final *-m is problematic.
In Kra-Dai, PAN medial *-N- evolves to *l, while *-n- evolves to *n. Compare *aNak ‘child’, Proto-Kra *lak, Proto-Tai *lɯ:k, Proto-Kam-Sui (Ferlus) *la:k 8; *buNum ‘air, weather, sky’, proto-Tai *C̬.lɯm A ‘wind’, Proto-Kam-Sui (Thurgood) *hlwɯm 1, (Ferlus) *(C)l/rəm A. Consequently, all the Kra-Dai words for ‘water’: Proto-Hlai (Ostapirat) *nam C, Proto-Tai *C̬.nam C (Pittayaporn; ‘C̬’ is a voiced consonant), Proto-Kam-Sui (Ferlus) *n-, Proto-Lakkja *num C (Theraphan) are cognates of *nanum, not *daNum.
There are no obvious reflexes of *nanum ‘water’ in MP, however it appears that the PMP word for ‘water’: *um-inum ‘to drink’ was reanalyzed from *mi-nanum ‘get water, drink water’ (here).
Tsuchida, S. (1982) A Comparative Vocabulary of Austronesian Languages of Sinicized Ethnic Groups in Taiwan, Part I: Western Taiwan. Memoirs of the Faculty of Letters, No. 7, University of Tokyo. Tokyo, Japan.