First of all, think about colours. You'll want to get a heap of party decorations in pale blue, dark blue and green, and then place these around the room. Balloons are great, but don't buy all one colour - mix them up. The only exception to this is to buy some pearl white balloons, transparent if possible, and then create bunches of these to look like bubbles.
Buy some ribbon, streamers or other hanging decorations and have these hanging from doors, over windows and from the ceiling if possible to look like seaweed.
If you can find some netting material then grab a blue or green tablecover and then place the netting over it to look like a fishing net.
You know those pasta shells you can get? Sprinkle a few over the table, or if you are near the beach you could always pick up a few there, but make sure you wash them! Don't forget the all essential birthday cake of course. If you're making your own, why not keep it simple by making a basic sponge sprinkling it with Demerara sugar and topping it off with Guylian seashell chocolates.
Do you have any other mermaid party decorating tips? If so please share them by adding a comment below!