Playable races

The simplest way to start: choose one of the races below.
The more complex way to start: choose your ancestry and culture separately, using the Ancestry and Culture expansion. You can also use the A&C expansion to mix ancestries. Note that in the Sealands world, this is the only way to use the DnD-standard races half-elf and half-orc. Thus you won’t see those in the lists below.
Races in boldface have definitely been seen in this setting before, but anything goes (probably). See also the World Encyclopedia entry on (some of) the Sealand's ethnicities and cultures. All info and stats on races is taken from http://dnd5e.wikidot.com/.
This page is a good summary of most of the officially published racesin DnD. The articles at that website are very technical, so to speak, so it's best to look there just for summaries and then look at dnd5e.wikidot for info.

Common races

• Dragonborn
• Dwarf
  • Hill Dwarf
  • Mountain Dwarf
• Elf
  • High Elf
  • Wood Elf
  • Dark Elf
• Gnome
  • Rock Gnome
  • Forest Gnome
• Halfling
• Lightfoot
• Stout
• Human
• Tiefling (demon-descended)

Sealands-specific races and mixed races (homebrew)

These are homebrew playable ethnicities; links lead to statblocks in the Sealands world

• Maveren (mixed sea elf and human)
• River dwarf (mixed wood elf and hill dwarf)
• Willowisp (homebrew fairy variant)
• Mountain elves
• Fedun (mixed human and hill dwarf)

Other published races

These are races from official DnD releases that I can immediately imagine fitting into the setting. But again, anything’s possible

• Centaur
• Deep Gnome
• Duergar (deep dwarves)
• Eladrin (fey elves)
• Fairy
• Goblin
• Hobgoblin
• Kobold
• Lizardfolk
• Orc
• Satyr
• Sea Elf
• Tabaxi, because doesn't like cat-people?
• Triton

Homebrew player races

I liked these enough to download them, at least; they may or may not have been playtested. Stats available upon request

• Ahrcoon (anthropomorphic raccoon) (other animalfolk possibly available)
• Fey races
  • Brownie (goblin variant; native to the Feywild)
  • Leprechaun (yep)
  • Feyling (mixed human and fey; good if you want to charm and enchant people)
• Awakened Undead
  • Skeleton
  • Revenant
  • Ghost
  • Ghoul
  • Mummy
• Jinx (halfling variant; brings bad luck (to others, mostly))