Not everything, but about 98% of the code is currently in BP, simply because prototyping in BP is much faster. Once I have more final idea of how the game systems should work, I will start rewriting the least performant parts of them in C++.
And yes, water and air units planned, but still quite far on the todo list. The main goal is to have very simple playable skirmish scenario, with a few buildings to build, few units to build, resource gathering and basic AI. Basically something a player can actually play and win. That will test if most of the game systems are in place and work properly.