I have tinkered with the sub building slightly and i found it rather annoying to not be able to test the sub with out having to start up a game when really you just wanted to see if a certain part of the sub worked. So it would be nice to have the ability to spawn in a character you can control and some what effect the environment around you. Here’s a list of option you can have:
1. The ability to enable/disable water so you don't have to worry about flooding when your just trying to see if a button does what you want
2. The ability to disable buoyancy as the player may just be trying to see if a automated system can handle a flood correctly
3. The ability to add obstacles in order to test how well the sub would fair against arming something or falling on to a rock
4. The ability to toggle God Mode (i think this one is obvious)
5. The ability to either restore the sub to its state before testing or retain any damage done to make a challenge sub

The last two abilities are already possible using the console (F3, type help) and the commands god and fixwalls / fixitems. As for the first three, I agree that it would be nice to have them. I don't see the point in not having them, and it would help with sub building.