If you are tipping $20 from each diver total for 5 days of diving then you are, IMO, undertipping. If you dive 2 tanks/day that's $10 per person PER DAY tip, minimum. 5 days = $50 PER PERSON at the very least. Doesn't matter if you're part of a group. Each diver is still getting the service each day.
I haven't done a live aboard but if I do I would tip 15-20% of the costs at the very least. And if $5/tank came out to more for the number of dives I did then I'd go that way.
If I've misread your post, I do applogize.
As for gifts, well, I have given things to people that we've gotten to know quite well as regular customers, but it has NEVER been in lieu of a tip.
I've had tipping conversations with a few DM's that I've gotten to know well, and there are people who genuinely do not realize that tipping on a dive boat is customary. Seems that if they come from a country where gratuities are customarily included for service (like at restaurants) then they are of the mind set that the price is all-inclusive unless told otherwise. If people come from places where travel is sold as one all-inclusive package (one price covers air, transportation, AI hotel, gratuities at the AI...) then they don't think to tip. The latter, surprisingly, even happens when tipping IS customary for them at home. I really think that dive operators should not be afraid to say "gratuities not included" on their pricing lists.