I've been on two boats in FL that had DMs on board but neither got in the water. I don't recall either giving a dive briefing... the captain did that. One didn't even really help with any gear so I guess they're there primarily in case of an emergency. One DID tie off on a wreck though but the other charter, I've only been out with them twice, did drift dives only so the DM never even got her pinkie toe wet. And then, of course, being a tipping country, divers are expected to tip. For what? I'm not sure but it's one of the reasons I'm turned off on tipping... (still do it though)
Vary's by op/locale.... but generally the more experienced divers tend to not stress crews, or require (or desire) DM assistance short of yelling "dive dive dive" and handling fins.
Our own maestro, ScubaKevDM is in the water and is generally a high touch operation.
Up in Riviera Beach, Little Deeper, the DM is in the water, and also insists on doing allot of the load in/load out along w/ a deck hand...
RJ Diving in S. Beach, and Parrot Island in Pompano, the DM facilitates paperwork, wreck tie in, and works the ladder helping people in and out, getting weight, etc.. These boats cater allot to shops\instructors, so they are a bit buzy w/ new divers working thru issues...
American Dream in FLL usually has 3-4 DM\hands assisting, not sure why, paperwork, wreck tie in, orange slices, etc...
Then there's the six packs... no DM, however will we manage? Oh yeah, we're 'real' divers...