Does anyone know when we will have more technologies for us to research?
Let me guess... in next version? Every version in the past added some technologies to research tree. That trend will follow. In 2.2 there will be UFO recoveries (currently work in progress) which enable some new UFO- and craft equipment-related research topics.
And when the files do come out, will it be a patch or files that I will have to place myself (which I doubt I know how to do it)?
New version means new installer to download.
How can we add our own devices to the tech tree?
base/0ufos.pk3, unpack, take a look at research.ufo file.
Many many things in the game just aren't implemented yet.
Yes. As usual, patches are welcomed.
are there plans for adding underwater missions and gear?
Yes. It is on my personal TODO list for UFO:Alien Invasion version 14.7.23 or something. Underwater bubbles will follow.
For that matter, how do we find out when the updates are scheduled?
Announcement on homepage, forum, wiki, sourceforge page. Also - since last version - newsletter.