Whats kinda wood for a floating stereo?

I'm building a stereo that i can go river rafting with 4 speakers a battery and a deck the question is what kinda wood will last the longest being exposed to water. it i plan to build it to be able to sit atop an inner tube so it wont be directly in the water. mainly it just needs to be able to last for a good 2 mos ive done the whole cooler thing i just wanna build something new