I'd be happy on a $5-$10 tip per tank. In general I'd rather be invited out for a drink or pupus than given a few beers. Every operation I've worked at the crew split tips, not always evenly, some place even split with the shop follks. Most of the crew are friendly with each other and actually make sure to split tips with each other. I think you can assume that tips are going to be shared. If you find that 1 crew member was great and the other(s) were awful consider leaving the tip in an envelope at the shop, most of the time that person would consider it a personal tip and not share. I also like the personal touch: hand the tip to the crew, even if a jar is there and maybe even take a second to explain why you liked diving with them; it can often make someones day.
If any of the following apply you should tip more:
The guide spends a lot more time dealing with or helping you than the rest of the boat. (This does not apply if you are very attractive member of the opposite (or same, whatever floats your boat) sex; you KNOW what I mean....).
You need rescue.
The Dm repairs something of your gear (that you lost).
You drop something over the side that the DM recovers for you (like keys, hats, towels, etc).
The Dm finds your HOLY GRAIL!
The guide shares his/her papaya (or anything he/she brought from home and is not provided by the boat).
The DM finds a piece of gear you lost.
The DM gets you to dive when you've freaked out and decided you 'can't' go.
You puke on the boat.
You puke on the DM/Captain.
You puke onto yourself and the DM finds something other than your towel for you to clean up with.
If when you are puking (see a theme here yet?)the DM makes sure you're comfortable, have water/shade and maybe moves you away from the engine exaust.
Basically if there is any sort of puke involved TIP EXTRA, someone has to clean that up!
If you clog the toilet on the boat (far grosser than cloging a home toilet, let me tell you!)
You create a large ammoung of extra work for the dm (again I think you know what I mean here..)
Basically if any the above apply you should probably tip more like $10-$20 per tank