Sometime during the week you'll get a letter/envelope or similar with the suggested tip amount. It's usually going to be 10-15% of your total trip price - less surcharges. It will be split among the entire crew. It will also be suggested that if someone does something you wish to reward privately, you can do that as well.
You generally pay it when you settle up with the ship's purser - most often the last day/night you're onboard.
Cash works better for them to distribute among the crew usually.
My normal tip on week long liveaboards is 20% since most that I've been on the crew members do at least double duty - most work the divedeck, clean rooms, take watches, serve meals and generally do whatever is necessary. And will usually DM/dive wth you if asked. I back that down if service is sub-par - so far it's been the opposite, the service has exceeded my expectations - I've been going on better trips lately...
Depending on your trip schedule, you can also do other things. On the T/C Explorer recently, the Captain and several crew members arranged for and accompanied a large group of us to dinner in port the last night - although they do it for every group - we paid for their drinks/meals.