I recently found a glitch that allows you to get out of the map and make a seamoth.
Step 1. Get a mobile vehicle bay or a seamoth.(This might work with prawn but that takes a long time to get so not speedrun viable.)
Step 2. Find a cave(works with both) and place the mobile vehicle bay so that it floats to the top of the cave. If using a seamoth just drive to the roof and make sure the seamoth stays there.
Step 3. Make sure the vehicle bay is tilted down and climb on. It will automatically kick you off and if you are in a good spot, it will kick you out of the map. If using a seamoth, jump in the seamoth but before you get in press E(or whatever button it is to get out) to get out. If you timed it right, you will be swimming outside the map.
Notes: There is a cave underneath the alien weapon platform island that I would use so that if you use the mobile vehicle bay inside the mountain then you can make a seamoth. For some reason, the cyclops won't make. But it doesn't matter because you probs won't be getting that. This allows you to go all the way down to the final alien base. Note that I have not done this but I see no reason you can't do this multiple times anywhere. I will post a video later.
Note #2 I found a fun glitch with the beacon. If you are standing directly on the beacon when you pick it up. You go flying.
This is a link to my example. I am pretty quiet because I didn't want to disrupt my house.