When diving from a boat does anyone give the crew a tip? If you do, how do you determine how much to give?

It's up to you, I normally do on a liveaboard. How much will depend on the country I'm in.

I always do, unless they've been a bunch of unhelpful bar stools (which hasn't happened yet!)

As Atlantic said - depends on the country. Maybe kitty in with other divers or ask your guide.

For example in Egypt if you are on a day trip , the tip is for the crew is included in the meal or drinks but if you do not tip them, usually the diveclub gives a certain amount per person on the boat anyway.....but be carefully with tipping , because if that person views you as a friend in that case you can offend them deeply, it all depends the way you give it.
But usualy you should only tip them if they have given you some special attention or help ...they will enjoy this.
On a life aboard ou have to decide by yourself how much you want to give them...but for sure they will appreciate this because it will help their families to live.

Was in 2 minds whether to mention this, but, on our last liveaboard in Egypt we all decided to give 100EGP each per envelope. There were 2 envelopes, one for crew & one for guides. A little strange to have one for guides but we did it anyway. After we handed over the envelopes to the captain he sorted them out and added them up. The crew then complain as there was only 1200EGP in total and called us stingy. We told them we had all put in 100EGP in each envelope so it should total 4000EGP. We added it up and it was 1450EGP! As we were all talking about it 3 of our group said they had put in English money and there was none present. The crew said there were no £UK in the envelopes, one of the guides went to the captain and came back with 3 x £20 notes. We were then getting a bit pi$$ed off and told them they should ask the captain for the rest of the EGP as well.

Sour taste in the mouth.

You have given it to the captain , so, he decides what to do with it and the crew should be happy with that.
It is not your problem anymore once given to the captain + 4000 Egyptian pound is a lot for them.
But the fact that they complain shows that they have been spoiled by other tourists........