I don't know if that's the real translation. I read a bunch of forum posts and articles and most of them said that it wasn't really Morse code, it was Tap code, and that it was very difficult to translate.
According to a video on youtube ( http://www.youtube.com/watch?v=QYDUp-m7GDo ) the code translates to "Hey,you never call, how d'you fancy going bowling ?", Which is an obvious reference to Roman from GTA IV and the fact that he will constantly call Niko to invite him for bowling throught the game.
After browsing the web little while ago I found something joinable to this.
When James Cameron went to the bottom of the Mariana Trench in his Deepsea Challenger submarine in 2012, he apparently found a mysterious door at the very bottom, which he managed to open. He didn't want to say what he found inside.
If we put these two together, would you see similarities? I mean, maybe the Underwater Hatch is not of human origin at all, despite the tapping code message heared from it in every 4 minute intervals between tappings.