Where can I get some good underwater rules that take into consideration combat, spells, missle weapon ranges, elemental effects (acid, sonic, fire, lightning, etc.)?
I *think* there are some in the Manual of Planes. I tend to extrapolate from some 2nd Edition sources on this (and haven't been wrong yet - thank you Sage for your recent fly spell underwater ruling that was a HUGE argument 2 summers ago ), and I think a 3rd party publisher put out some rules.
You may find them fairly stripped-down. There are no modifiers for for different energy types, for example (other than fire, which is more-or-less banned underwater).