well since you're going to a water park, i think it would be kinda hard to do a theme but you could do a hawaiin theme or an under the sea type thing
my themes have mainly been princess themes lol
when i turned 14 i did a blue and green theme
everyone wore blue and green, and some people really dressed up so it was kinda fun. here's a pic:
and then for my 15th it didnt really have a theme but i had a princess cake [sorry i like pictures

]:
and then when i turned 16 i had a laser tag party with a runway cake [i love modeling and all that] and i also had a red carpet [dont worry, it was cheap]
and then for my 17th i went to
disneyland so it was obviously disney themed
[notice the disney shirt, disney necklace and disney button thing. i also had a disney bag

]
for my brothers 16th or something like that we did a hawiian theme, same for my cousins birthday last year. for my brothers we bought coconut type cups that had a straw, had a pinata and leys and all kinds of things
for my cousins, we got her a grass skirt and a coconut bra along with a dolphin backdrop and some other things
i also had one friend who had a candy theme
she had candy pillows hanging from the ceiling and a candy bar
in the end, it all depends what you're into
you could do a superheros theme
a theme from your fave movie
one of a video game
i know someone also did a platnium one i think?
everything was like gold and silver
so whatever you're into works