Is this encounter in ghosts of saltmarsh ridiculously deadly?

In the Ghosts of saltmarsh appendix A, "Of ships and the sea" one of the pre-created underwater locations (the wreck of the marshal) has the following feature:

I’m hoping to run one of the suggested adventures in this area (the one for lvl 5 characters), but I’m struggling to see how this feature is supposed to work. As in this question the PCs will have to make 8 saves to avoid being charmed, so even with the low DC of 11 (and the elf in the party having advantage), the chance of at least one failure is high. Once they are charmed they will get no further saves if they just swim straight towards the harpy that’s charmed them and then that harpy can kill them without them getting another save (as long as it’s the harpy they’re charmed by). To me, this seems like a death sentence to any characters that haven’t make preparations in advance (like stuffing their ears or using the Silence spell).

Am I reading this wrong? It feels mean to just say to my players "roll 8 wisdom saves". Which makes me think I must have missed something here.