What waterproof temperature sensor do I need to measure water temp using an arduino mega?
The DS18B20 used by the Sparkfun sensor can be used with or without external power supply. The Sparkfun seems to have three wires, so that will be ground and data and Vcc. (so far for a 1-wire interface!).

  

Black is ground, red is Vcc, and white is data (per comments here).

The data line will be pulled high by the resistor when idle. You're not supposed to make the I/O high! If you want a high level, switch to high-impedance input. To make the line low, set to output and set the level to "0".
May sound a bit complicated, but once you set the output level low, you don't have to touch it anymore, and set high/low by just changing the Data Direction bit.