underwater hell or otherworld

[Stefan Baums]

Dear Jonathan,

the word pātāla (both Skt. and Pali) may mean what you are looking
for. MW has:

   one of the 7 regions under the earth and the abode of the Nāgas
   or serpents and demons [...] sometimes used as a general N. for
   the lower regions or hells

with, mostly, Epic references. PTSD is more cautious:

   proclivity, cliff, abyss

but the Pātālasutta (SN IV 206–207) does suggest an interpretation
as ‘hell’ by reinterpreting it as mental suffering:

   assutavā bhikkhave puthujjano yaṃ vācam bhāsati atthi
   mahāsamudde pātālo ti taṃ kho panetaṃ bhikkhave assutavā
   puthujjano asantam asaṃvijjamānam evaṃ vācam bhāsati atthi
   mahāsamudde pātālo ti. sārīrikānaṃ kho etam bhikkhave dukkhānaṃ
   vedanānaṃ adhivacanaṃ yad idam pātālo ti. [...]

More recently and apparently with reference to this sutta, the
word was understood as ‘hell’ by Dhammananda Mahathera in his
essay “The Buddhist Concept of Heaven and Hell”:

   When the average ignorant person makes an assertion to the
   effect that there is a Hell (patala) under the ocean he is
   making a statement which is false and without basis.

   http://www.buddhanet.net/budsas/ebud/whatbudbeliev/303.htm

Hope that helps,
Stefan

-- 
Stefan Baums
Asian Languages and Literature
University of Washington