Once I found out about Easy Place within Litematica, building big builds became much easier. But going under water sends me back to before Litematica existed.

There should be a way to Easy Place blocks while under the water so we can build huge aquatic bases or the like without having to place one block a second.

This is now fixed in the 2021-09-06 build.

Oh sweet! How do I get it?

By downloading it from CurseForge like the previous version you have.