Tip or don't tip, it will always be up to the individual diver. After working on the same liveaboard for eight years I can say that those guests that tipped well were always appreciated. When they came back (and 70% of our guests were return guests), it was remembered and and crew went out of their way to go the extra mile. While I can't speak for day boats, I can say that liveaboards offer lots of service aside from dive deck operations--cleaning cabins, serving meals, laundering towels & linens, carrying luggage to and from the boat, the list is endless. These are DMs and instructors, but they are doing work outside of normal diving ops--the things they never told you about when you were working towards that instructor rating. This doesn't include the special services, like when a guest accidently drops a piece of gear over the side and we jump in and retrieve it (I've retrieved DOZENS of pieces of gear, including a $5000 camera from way beyond recreational diving limits), or when a camera isn't working right and we use our expertise to fix it, or when we find out it is a special occasion (birthday, 100th dive, etc.) and we use our "off" time to bake a cake and sing to make the occasion memorable, or when a guest shows up with a reg that hasn't been serviced in years and we fix it for no charge. These things certainly go beyond providing transportation to dive sites and assuring a safe dive environment.
I've chatted with guests numerous times about tipping and one subject that frequently comes up is that the guest suggests the tip SHOULD be included in the cruise fare. I try to explain that as an employee, I would love that, but IF the tip was included in the cruise fare, there would be no incentive to offer top-notch service. It's simply an incentive to go above and beyond.
There have been times where guests have tipped the crew $20 for a week of top-notch service and the crew gets angry, but I've always tried to explain that maybe that was all the guest could afford and this might have been a once-in-a-lifetime experience. I urge them to keep giving the great service and they'll come back, hopefully when they can afford a bigger tip. Karma is a wonderful thing.