All the times I have found a Megalake biome it didn't had even 1 block of water, it was always just a grassy field with patches of sand. And because for me it always generated without resembling a lake at all, I have even maintained the english name in the pt_br translation because I wasn't sure if it was supposed to be a lake biome or not.

And well, I've heard people saying that Optifine is "incompatible" with world generation mods (especially biome ones), and I have Optifine installed because otherwise Minecraft would be near unplayable for me. So maybe it has a relation with this weird Megalake generation?

This was a bug in the world gen, but it was fixed in b832733!

that's weird ...

maybe I'm running a outdated version? I'm gonna check this

I checked it and I'm running the most recent version. maybe the issue was fixed but the update containing this fix wasn't released yet? well, but there's a issue about Megalake biome that is pretty recent ( #100 ), and it can be observed that the lakes actually generate there. so what could be the issue there then?

You are right, the fix is not released yet! Right now, lakes can spawn, but they are almost impossibly rare, which is why that other issue could be found.