Invitation Help!!!!

For our reception we will be having the coctail hour on a boat that leaves the dock. Our guest are invited at 6:30 and the boat leaves the dock at 7:00.

How should I word the reception card so that it says something about the invite time of 6:30 and that they must be there by 7 or the boat will leave them behind?

Thanks

You can always put something like:

Cocktail Hour at 6:30pm

Boat Name sails promptly at 7:00pm so don't miss the boat!