After seeing some entries I can say that the theme is very narrow. It's not a bad thing dont get me wrong. It's the reason why I entered this jam.
Clear theme gives you time for research on same-themed games, thier art and execution before jam and not while it running.
You can however broaden the theme into 'Underwater Themed Games' (dont forget to give people some examples of what types of games they can do) or something similiar because right now people more or less gravitate towards the same idea of an adventure/mini-game when they think of submarines.
As for time constrains, a week is perfectly fine.Thanks for the jam. :)