There are several ways to have the tubing exit the hull. The least expensive and easiest is to drill a hole slightly smaller then the tubing your using for the motor/esc cooling and have it directly exit the hull (see below picture). Make sure its well above the water line so that when your boat is running you will be able to tell if the system is working. Remember when installing this type of water cooling system use the least amount of tubing as possible. This way the boat will weight less and the water will flow faster. Also no sharp bends in the tubing and keep the tubing away from spinning motor couplers and drive shafts.

          One of he other ways is using a small piece (about 3/4" long) of brass 1/8" diameter tubing and epoxy it into the hull. Then on the inside just slide the tubing over the brass tubing. You may want to use a small zip tie over the tubing to insure the rubber tubing doesn't slip off the brass tubing!

          One of the nicest looking ways to have the water exit the hull is an Anodized Aluminum fitting by Aeromarine (below pictured), But if your on a budget this is one of the places you can save $5 dollars and go with one of the other above methods.

Installing a Water Pickup
 

          On the Aqua Jet rudder & lots of custom rudders made today, there is a water pickup built right into them. This is the simplest way to pickup water for your cooling system. Below is a picture of the Aqua Jet, it has a brass tube built in that you attach the rubber tubing of your input side of the cooling system.

          If the rudder doesn't have a place for a pickup, something like the below picture may be added. This type of pickup is designed for a surface piercing propeller. Because the top half of the propeller will be out of the water while at full speed, this type of pickup will cause less drag then having the pickup dragging in the water. The prop wash that comes of the top half of the prop will be forced into the tube. 

          To start this project you will need a long piece of brass tubing. How long, depends on how far out from the transom your propeller is. Next you will need to make the tube in the shape of a "j". If you have never bent tubing check out this page, How to bend tubing. Once you have the piece of tubing bent , cut a 45 degree angle (see below picture) in the tubing to help it catch the water that's flying off the propeller. To mount this in the hull drill a hole the same size as the OD (outside diameter) of the tubing. Use a quick drying CA to temporarily attach the brass tubing to the hull. Then use a quick drying 5 minute epoxy to permanently attach the tubing. Don't be cheap with the epoxy there will be lots of stress on this part, whether it be water flying off at 30mph or a crash. If this breaks off, the hull is breached and it wont take long before the boat sinks! This is one of the reasons I make the hole above the water line for the tubing.

These aren't the only ways to make a water pickup, but they are the most popular and will cause the least amount of drag!