it's kind of "it's owed to me" mentality even though the individual didn't supply any extra service above which I payed for.
You're ignoring the simple reality of those situations where tips are customary:
You really have NOT "already paid for" the services for which the tip is intended.
Forget for the moment whether the whole idea of tipping makes sense in general. The fact of the matter is, it's a reallity in many situations.
I know this will sound like "semantics" but since you know that the DM on the boat is not getting paid by the boat, you know you
haven't paid for the services rendered by the DM.
If you would like those services to be included in the fee that you pay to the boat, I'm certain that you realize that the fee you pay to the boat is going to go up, right? Probably more than what you would have wanted to tip. (A LOT more than you would have wanted to tip, apparently.)
Someone suggested above that the boat should pay the DMs $100-$150 bucks. Let's use that math for the boat I'm on. With three crew members that would be $300-$450 in extra expense the boat would incur. That will be reflected in an increased price of a charter.
We carry an average of 15 divers, so that would come to $20-$30 extra for the price of a charter. Since most people here - tippers and tip-ees - seem to indicate that a $10-$20 tip per charter is "the going rate" it would seem that "including" the tip in the price of the charter could result in a net increase of 200% in terms of what you'd be paying for the same thing.
Be careful what you ask for, you may get it.