Dorkhedos,
The speed on an engine is controlled by a carburator which is controlled by a servo and linkage. It's often a fool-proof way to control the speed but occassionally a newbie gets tripped up with the "keeping the speed consistant in a turn process". This is caused by not cutting the throttle cable extra long and then securely mounting it to the engine.
Food for thought. Is there a club or a member of this site in your area? I'd strongly recommend that you hook up with one of them before you decide to do this. For one, a lot of places are becoming nitro unfriendly which is where electrics come in. The lack of noise is more acceptable. Two, tunnels can be very finicky, hard to setup properly and frustrating. However, they also are the most realistic looking and are exciting to watch and run.
If you want ideas on how a radio box is setup (because now, you have to worry about keeping water away from the radio) look through the Outboard forum. There are quite a few posts with images on there. I think you'll find it very hard to get even a used boat & engine for $150 but it's not impossible.
Till later, rock on and welcome to the hobby,
Snowdog
