I'm 90% sure that previous editions ruled that casting lightning bolt underwater turned it from a line AoE into what we would now term either a burst or a cone AoE. Not sure if there's anything official that updates that to 3rd Ed though.
Second edition DMG, page 79:
Spells are also affected by the underwater world. Not surprisingly, fire-based spells have no effect unless cast in an area of free oxygen (such as a domed city).

Electrical spells conduct their energy into the surrounding water. Thus, a lightning bolt originating 60' away from the caster acts like a fireball at the point of origin.
(note that, in the 2nd edition rules as written, a lightning bolt could originate anywhere the caster felt like, within range. But almost everyone played with it originating at the caster's fingertips, so that's what they officially made it do in 3rd.)

This doesn't say anything about what other electrical spell would do, such as Shocking Grasp. I suspect that most 2nd edition DMs would have said that Shocking Grasp underwater acts like Burning Hands, and that Chain Lightning also acts like a fireball, but with better damage.