Boats are wonderful things to make when you're getting into the RC hobby. The electronics and control systems are very simple and when it comes to design, the sky is the limit. A horrible design will not lead to the whole thing crashing down, such as in the case of an aircraft, and if it sinks (or capsizes, such as what happened in my case initially), the only replacement part I needed was a new Electronic Speed Controller. I built this after my ill fated attempts to build a tricopter and all of the parts were taken from that build, so improvisation is the name of the game. The main aim is to get it running as quickly as possible so you can extract maximum run time and you can make as many iterations as you want. Plus, unlike a multirotor, the battery life is phenomenal as it doesn't need the full power output of the motor to just potter around, and it has only one motor! So as you can see, I've used household materials as much as possible.
1 L Bottles
DT750 Outrunner brushless motor
EL cheapo RC TX/RX Combo (Avionic RCB6i from rcbazaar.com)