It's not tethered. They release the sub and let it sink, then have to send messages from the surface of their location to guide the sub. Some reporter went down to the Titanic last year and he said in his story that the only way they communicate with the surface vessel is through TEXT MESSAGES...?!? So besides all the other red flags (can't open from the inside, no sonar equipment, a vessel controlled with a LITERAL video game control, etc.) betting my life on a text message getting through would be a big WTF to me.
Not to make a joke, but we can't always get a text to go through at DWS, and it's on the surface of the earth.