Under the sea themes are different from beach themes and nautical themes. With this, create the atmosphere for your guests of being thrown deep down into the waters: flush the entire room blue to give the feel of being immersed in water (place up lighting around the perimeter of the room and if you’re fancy, even do blue overhead lighting with white spot lighting).
I love how you can hang ceiling decor to look like seaweed-even strands of crystals can give this effect and will add a touch of bling and glitz to the space, similar to the way sunlight looks when it hits water.
You can choose many invitation options that will work but remember to make sure it has elements of underwater life to it: water mixed with fish, or sea horses, etc. Don’t just use a shell or anchor as that would represent more beach theme or nautical theme.
You can also choose to be fun and bright with your cake or go more traditional by keeping it white and adding brightly colored sea creatures and elements.