I hope the pack has mermaids

I think Mersims would work fine in an EP as there's not as much variation in lore as there is for Vampires etc. Their potential powers/abilities aren't as vast in scope. They could be implemented with a standard set of abilities like aliens, E.G:

• Human and oceanic forms
  
• Dive longer because they breathe underwater
  
• Charismatic Siren-like powers (could be platonic or romantic)
  
• Power over water or sea life (create waves in pools, summon sharks)
  
• Better at finding rare underwater collectibles/fish
  

I hope they have a tie in with the Singing skill if you also have CL.