The version number jumps to 18.3.1.C.0.21 form the earlier 18.3.C.0.40. That old version introduced a sound delay bug, which has been fixed with the new software. There were also reports of fast battery drain, but it's not clear if that has been fixed yet.
The Xperia M2 Aqua is a waterproof variant of the base M2, but there are no reports of it getting the update just yet. Sony released open-source builds for the 18.3.1.C.0.21 build for all M2 variants, including the Xperia M2, about a week ago so the rollout should be quick.
There's no word from Sony on a Lollipop update for its lineup outside of the premium Xperia Z family.