King Triton was a figure of Greek mythology; the son of Poseidon and Amphitrite. According to the myth, the family lived in a palace of gold under the sea. Triton used sea horses and sea monsters to travel the seas, and created weather by blowing on a conch shell. He had the upper body of a human man with the tail of a fish. King Triton also was interpreted in modern times in the Disney movie, "The Little Mermaid."
Find a flesh-coloured shirt or body suit. A long- or short-sleeved skin colour T-shirt also will work. A costume shop also might have a body suit or at least a flesh-coloured top resembling the upper body of a human man.
Call your local costume shop to ask if they have a fish tail appropriate for King Triton or for a mermaid. Make sure to ask about the fish tail's length and compare it to your height so that it doesn't drag on the ground. Purchase one that is best suited for you.
Ask the costume shop about long white or grey wigs with moustaches and long beards. If you have old toys, such as dolls or horses, with long white hair you can cut off this hair and try to stick it to your face with glue to save some money.
Purchase a conch shell from a pet store or aquarium to complete the costume. If you can't find a conch shell, you can substitute a triton and claim that you are the King Triton from the Disney movie.
Pretend to be blowing on the water to create waves or riding waves to create the final effect.
It probably does not matter much whether you tell people that you are the King Triton of Greek mythology or the King Triton from the Disney movie. Many people are not familiar with Greek mythology and probably wouldn't understand your reference. However, the character from the Disney movie will receive a much wider audience. You may also attempt to dress up as a statue of King Triton, in which case you want to be covered in grey or silver to impersonate stone.
Given that the King Triton from "The Little Mermaid" is more recognisable -- especially to children -- do not worry too much about being too historically accurate. However, be prepared for the few literate in Greek mythology to lecture you on accuracy. There's always one.