Underwater base building will be partially in at Early Access launch: you won't be able to build vacuum sealed underwater bases, but you will be able to build in underwater air pockets that are like mini-caves embedded in reefs along the ocean floor. However, building out the technology tier that will include vacuum sealed structures is a post Early Access priority. As for guns and weapons, they can be found in loot crates but also crafted from blueprint items, or engrams that you learn via skill progression.
Depending on the weapon it may require many kinds of components that also need to be crafted, rather than just raw materials. There's also a weapon attachment system so you can add scopes, sights, flashlights, and other accessories onto a gun. As for how many... too many to easily list off the top of my head but we got all the obvious types covered already (i.e. sniper rifle, machine gun, pistols, shotgun, rocket launcher, grenades, tripwire explosives, remote det c4's, among other weapons) and will continue to expand
-Drake
