I tip the DM 5 to 10 per tank, depending on service. If I have a guide, that's an extra tip that is usually pretty generous.
The idea of a mandatory tip kind of rubs me the wrong way and all I can offer is what happens when I see it in restaurants.
1. After implementation, service usually goes from "excellent" to "ok".
2. I usually tip 20 percent in a restaurant. (two kids working through school waiting on tables makes you do that). If there's a mandatory tip included on the bill, I never exceed the amount they charged, so the waitstaff actually ends up with less than they would have gotten...at least from me.
3. The restaurant has to be very very good for me to go back a second time. I'd much rather they just increase the prices on the menu and pay their staff better.
4. I can't see my reaction to having a tip added on for me at a dive operation as being any different.
By the way, I've been on Captain Gary's boats a few times (though I doubt he'd remember me). Each time was an excellent experience, and of course, I tipped accordingly.