in order for someone to establish a claim in neglience, they have to prove
certain elements. there are four classical elements (duty, breach, proximate cause, and damages) but they can be simplified to three: duty, breach, damages.
so, in order for anyone to be found negligent, the other side has to prove
that you owed someone a duty, that you breached that duty, and that
they were damaged as a result.
just being in a boat where someone gets hurt is not enough to
create a duty of care towards that person, even if you are a DM
however, if you are diving with a buddy and they get hurt (you owe
them a duty of care because they are your buddy), the fact that you
are a DM will be used against you to show that you knew better and
should have acted better -- i.e. in proving the second element,
breach.