On my local gulf charters I have started to tip $20-30 (approx $10 per tank). I do most of the set up and hauling of my gear myself, but appreciate the DMs help during the day. So of the dives are less formal and the least I can do is buy a beer or two at the end of the dives (at the dock of course). I try to do the same on other charters in the US. That said, I offered the same while I was in Jordan on the Red Sea and they thought I was crazy as it was 'just part of the job". It is going to depend on where you are. US and tourist areas, it is definitely viewed as more "normal" or required.