Under the Sea Party Ideas: The Complete UK Guide

An under the sea party works brilliantly as a broader alternative to a mermaid party — it opens up a wider range of characters (fish, crabs, sharks, octopuses, sea turtles) while still allowing the sparkle and ocean colours that make the theme so appealing. It also tends to work well for both girls and boys, whereas mermaid parties skew heavily female.

This guide covers under the sea party decorations, food, games, party bags and costumes — practical ideas for a UK party that looks great without requiring professional event styling.

Under the Sea Party Decorations

Colour palette

Ocean blue, turquoise, aqua, sea green, sandy yellow and coral. Add silver and white for sea foam and light-on-water effects. This is one of the most naturally beautiful palettes in children's parties.

Key decoration elements

• Balloons — turquoise, blue glass, ocean blue, sea green and coral biodegradable latex balloons in a massed arrangement. Trailing blue ribbons create a flowing underwater effect.
• Hanging jellyfish — round balloons with crepe paper or ribbon strips hanging underneath, in blue, white and coral. Hang at different heights from the ceiling.
• Tissue paper sea anemones — large tissue paper pom-poms in coral, turquoise and purple, arranged low on the floor or on tables
• Net and shell backdrop — a fishing net (available from garden or seaside shops) draped behind the party table, with shells, starfish and paper fish tucked in
• Underwater photo backdrop — blue streamers cut into strips and hung to create a "wall of water" for photos
• Sand tray — a shallow tray of sand with shells, pebbles and small sea creature figurines makes a beautiful centrepiece and doubles as an activity

Under the Sea Party Food Ideas

Savoury food

• Fish sandwiches — tuna or prawn mayo on brown bread, cut into fish shapes
• Crab claws — carrot sticks labelled as crab claws, with red pepper hummus "crab body"
• Octopus hot dogs — mini hot dogs with the bottom half cut into eight "legs" and cooked to curl outwards
• Coral reef crudités — broccoli (coral), carrot sticks (sea fans), cucumber rounds (sea stones), cherry tomatoes (sea urchins)
• Seaweed salad — cucumber strips or green noodles labelled accordingly

Sweet table

• Under the sea cake — blue frosted layer cake with wave effect frosting, edible sea creatures, crushed digestive biscuit "sand" at the base and gummy fish pressing through the sides
• Jellyfish cupcakes — blue frosted cupcakes with gummy worm tentacles pressed into the top
• Sand and shells dessert cups — crushed digestive biscuit in small clear cups with blue jelly "water" on top, shells made from white chocolate
• Fish bowl jellies — blue jelly in round-bottomed glasses with gummy fish set inside
• Starfish shortbread — shortbread cut with a star cutter (the closest available to starfish shape)

Drinks

"Ocean water" — blue lemonade or blue sports drink with edible glitter. Label as "Deep Sea Potion" or "Mermaid Lagoon Water."

Under the Sea Party Games and Activities

Ocean creature hunt

Hide plastic sea creature figures or laminated fish pictures around the party space. Children collect as many as possible and identify them. Award a "Sea Explorer" certificate to everyone who finds five or more.

Pin the fin on the fish

Classic blind pin game with a large fish outline. Works for all ages.

Fishing game

Attach paper clips to card fish and use magnets on strings as fishing rods. Children fish for their own small prizes. Simple and consistently popular with under-7s.

Sea creature craft

Make paper plate fish, jellyfish bags or octopus sock puppets. Paper plate fish: paint a paper plate in ocean colours, add a triangle tail cut from another plate, stick on a googly eye and paper fins. Quick, colourful, take-home result.

Underwater freeze dance

When the music stops, children must freeze in their best underwater creature pose — a starfish (arms and legs out wide), a jellyfish (arms floating slowly), a crab (sideways walk and claws), a fish (pucker lips and wave arms like fins).

Shell identification

For older children (7+), a shell identification activity using a simple guide sheet is a genuinely educational party activity. Provide 10 different shells and a matching guide. Award a prize to whoever identifies the most.

Under the Sea Party Bag Ideas

Browse our under the sea collection for themed supplies, and our mermaid party range for the crossover items. Under the sea party bags work best when they extend the ocean exploration theme — activity-based, curiosity-focused items rather than generic plastic sea creature toys.

What to put in under the sea party bags

• Sea creature discovery kit — a small identification guide and magnifying glass for rock pooling
• Grow-your-own sea creatures — expanding foam sea animals that grow when placed in water
• Ocean notebook — for recording ocean observations and drawing sea creatures
• Sea creature temporary tattoos — the nature/animal tattoo range includes ocean species
• Shell collection bag — a small organza bag with a few real shells and a card identifying them
• Blue or turquoise treats — gummy sea creatures, blue candy or ocean-coloured sweets to complete the bag

Browse the full party bag fillers range and eco party bag fillers for plastic-free alternatives. Present in a blue, turquoise or kraft paper bag with a shell decoration or fish sticker. "From the Ocean Deep" label if you want to go the extra step.

Under the Sea Party Costumes

• Mermaid — tail skirt (available cheaply), sparkle top, shell accessories
• Shark — grey clothing, shark fin attached to back, teeth painted on
• Crab — red clothing, claw gloves
• Octopus — purple clothing with stuffed fabric tentacles attached to a belt
• Fish — any bright colour with fin-shaped card wings attached
• Deep sea explorer / diver — black wetsuit-style clothing, goggles, flippers. Great for children who refuse costume parties.

Under the Sea vs Mermaid Party

If you're deciding between an under the sea party and a mermaid party: under the sea is broader, works for mixed groups, and gives you more character variety to play with. Mermaid is more specific, more sparkle-focused, and works particularly well for girls aged 4–8. You can always run an under the sea party with a strong mermaid element if you want the best of both.