Alien are the creatuers that are swimming outside the submarine almost every alien in Barotrauma is aggressive, some of them can enter the submarine. It depends on the difficulty how strong and how much health they have if the difficulty is really hard they will break in to your submarine in seconds. If you enter some alien ruins you may find many aliens but most likley that you will find is called the fractal guardian it's pretty aggressive if you ask me. There are many weapons that you can deffend yourself agianst the aliens, but beware of the Husk if it bites you, well then congratulations you got infected with husk parasite, don't worry if you have clayexandie on your submarine you are still saveable well not if you reached stage 3 if you reached stage 3 well then get out of the submarine (without a diving suit) and die, or you can attack your friends if that thing burst out of your mouth. The endworm is the largest creature known in barotrauma it hates submarines but like humans what i mean by that is that it will sink your submarine in a second but if you are outside and you met it well you are fine. The watcher is also passive if you don't provoke (shooting it and etc) it won't do anything. The coelanth well you won't really meet this creature it lives in the abyss but sometimes it rises to the level well you can maybe see it if your are lucky but i promise you won't be happy to see it. If you have to visit an alien ruin it is recommended to bring a harpoon gun with you a revolver is also good but not so effective and you can always get your harpoon back.