I think underwater ruins/sub wrecks should be able to be generated in missions, so sometimes the crew may divert themselves to explore what treasures they may hold.
I think supplies could be in a crate within some of the ruins to help the crew if they get stuck, or could use extra supplies.

Some ideas for underwater ruins I have are:

Sunken outposts- old abandoned outposts hanging off the edge of a underwater cliff, just waiting to be pillaged of the goods of its former gibbed inhabitants.
Sunken submarines, with uranium, harpoon guns or ammo..But beware, there may be crawlers or husks, mudratpors, or even a terrifying thresher guarding it.
Some of them could be semi working, the outpost either inhabitated by hostile human AI, trying to get a meal out of you, or traders, offering a way to exchange supplies-
Example: Uranium for some ammo, etc.
Maybe the sunken subs could even get working, with advanced engineering knowledge of wiring and such, and somehow pumping out the water. They could act as a cargo transport for the mission or as a combat escort for one mission. This would be VERY useful if you somehow can get it up and working when you may have to hunt an Endworm..

There is an infinite amount of possibilities for kinds of ruins, not just limiting it just to one layout, but multiple prefabs, unless you can make it a dynamic generation. This also would pave the way to more interesting ruins made by modders, since the framework for it would be made.

Some of these ruins could have 'distress' signals running, depending on what RNG decides.
I'll edit this if I brainstorm any more ruin ideas, but please, reply what you think, or with ruin ideas.
This might not be possible until late in dev, since ruin generation might be difficult with the dynamic terrain?

I’m going to bring over a tradition from the astroneer forum
+1

Maybe the items wouldn’t be just in a box at the end but scattered around the sub from on the floor to in a box, and maybe even in a corpses pockets