To get a unique name I personally like to take two names that I like and somehow mix them together like ummm.... Dara + Malaika = Dalaika or Camael + Salacia = Camacia or slightly change something about the name like Finn to Finan or so

Also I always have a long list of names (just in case I might need one real quick, it's hard to find fitting names)

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Here's my mermaid- themed list so far:

Popular Mermaid names:
Sirena
Nixie
Morgana
Melody
Myra
Cleo/ Clio
Lorelei
Destiny
Arielle
Delfin/ Delphin/ Delphine
Marissa
Lola
Oceana
Pearl
Adella
Aqua
Nora
Coralia
Miranda
Venus
Undine
Aquata

Popular Merman names:
Neptune
Triton
Dylan
Nalu
Dylan
Dax
Poseidon
Arion
Caspian
Finn/ Fynn
Jona/Jonah
Murphy
Reef
Ren
Strom


Stars/star signs: (gender neutral)
Aquarius
Sirius
Castor
Pollux
Procyon
Auriga
Orion
Bellatrix
Capella
Andromeda
Cassiopeia
Delphinus
Atair
Lyra
Arcturus
Antares
Rigel
Mira
Phoenix
Pisces
Cygnus
Polaris


Gemstones: (gender neutral)
Ruby
Emerald
Sapphire
Topaz
Tourmaline
Jade
Opal
Onyx
Pearl


Mythological water deities:
female:
Acionna (celtic)
Belisama (celtic)
Danu (celtic)
Sinann (celtic - irish)
Sequena (celtic)
Vedenemo (finnish)
Vellamo (finnish)
Amphitrite (greek)
Ceto (greek)
Brizo (greek)
Doris (greek)
Galene (greek)
Najade (greek)
Nereide (greek)
Electra (greek)
Thalassa (greek)
Scylla (greek)
Thetis (greek)
Triteia (greek)
Ran (norse)
Sága (norse)
Nix (norse)
Nuying (chinese)
Mazu (chinese)
Nāmaka (hawaiian)
Danu (indian)
Yami (indian)
Sedna (inuit)
Laumė (lithuanian)
Nanshe (mesopotamian)
Ahurani (persian)
Salacia (roman)
Atabey (taíno)
Ak Ana (turkic)
Nyami Nyami (tonga)

male:
Atlaua (aztec)
Lir (celtic- irish)
Nodens (celtic)
Ao Kuang (chinese- Dragon King of the Eastern Sea)
Ao Qin (chinese - Dragon King of the Southern Sea)
Ao Run (chinese- Dragon King of the Western Sea)
Ao Shun (chinese- Dragon King of the Northern Sea)
Sobek (egyptian)
Daucina (fijian)
Ahti (finnish)
Alpheus (greek)
Achelous (greek)
Nereus (greek)
Oceanus (greek)
Pontus (greek)
Poseidon (greek)
Proteus (greek)
Thaumas (greek)
Triton (greek)
Kanaloa (hawaiian)
Aruna (hittite)
Varuna (indian)
Paricia (incan)
Alignak (inuit)
Nootaikok (inuit)
Ryūjin (japanese)
Suijin (japanese)
Ikatere (māori)
Tohora (māori)
Chaac (mayan)
Abzu (mesopotamian)
Marduk (mesopotamian)
Aegir (norse)
Njord (norse)
Neptune (roman)
Veles (slavic)
Olokun (yoruba)

Some angel names I found: (great for mixing up)
Ramiel
Cassiel
Camael
Eremiel
Gabriel
Haniel
Raphael
Jehoel
Jophiel
Muriel
Raziel
Remiel
Uriel
Zachariel
Aavan
Adire
Leoc
Micah
Nanta
Navaa
Ona
Sariel
Vasa
Zedekiel
Laila/ Layla/ Leila
Malaika
Dina
Dara


My Mix-up's: (I take syllables of two different names and mix them together to create new names)
Veliel
Neriel / Nariel
Sana
Malene
Dalaika
Siquena
Thana
Emix
Nyanshe
Tohac
Belira
Marla
Varicia
Yaruna
Noomé
Dales
Casmo
Jadonna
Malu / Malou
Aone
Finan
Opaz
Torelle
Dareia
Caline
Laiac
Valir
Aquiel / Aquielle
Oceara
Camacia
Camaika