Blue dragons breathe lightning bolts. Underwater lightning bolts burst in a sphere. Caster or creature, a lightning burst centered on you sucks without being immune / metallic. A breed of dragon which hunts and takes lair underwater isn't suiting to any current breath weapon, especially lightning. Try Control Water for devastating effect.
Sure the original was based on a little known tale about splitting the sea, but it covers water vortex inhalations and water spout exhalations too. Consider it a creature specific variant.
So you ask for a retcon removal of Bronze Dragons' breath?