Ok. I did an experiment.
I sent 8 sims swimming in Mua Pel'am neighborhood (one of them was a vampire). I tried to keep them together, but they kept getting caught in currents. Using cheats I maxed out their needs and froze them.
They had many shark encounters. Vampire was first to drown due to their vampire energy need being empty (sunlight you know). After many hours sims started to be pulled underwater by a black mass.
Every time a sim was randomly pulled underwater by black mass, the victim drowned right after. When I looked at sim's moodlets, after resurfacing sims got an uncomfortable moodlet (instead of tense when a mermaid summons the ocean threat). That moodlet immediately disappeared when the sim began to drown. Waited until every sim had drowned, all caused by that black mass.
So shark encounters are harmless (and not rare). Being pulled underwater by black mass is rare and will result in death (I don't know if sim always drowns after it or not) unless it was summoned by a mermaid.