While the boat was on the trailer, he put a garden hose into the drain hole and added water to the bilge. He then reinstalled the drain plug and watched for any signs of leakage in the bottom of the boat and around the drain plug; there were no leaks.
I don't advocate using this method for tracking down leaks. Water weighs 8 pounds per gallon. A 16-foot boat can easily hold hundreds of gallons of water. If the weight of the water exceeds the capacity of the trailer, you could possibly blow out the tires or collapse the frame and springs.
Slow water leaks are sometimes hard to find. It is difficult to see down into the bilge compartment, especially in the bright sunlight. One method that I have used with great success is to wait until late in the evening, just before sunset. I make sure that the bilge is dry first, then I launch the boat very carefully so as not to splash any water over the transom into the bilge. I use a handheld spot light to illuminate the bilge and search for water leakage.
In the dim light of dusk the spot light will make finding leaks much easier. It may show up as a flowing leak or it might just be a trickle of water. By following that leak back to the source, I can determine what needs to be repaired.
Anything that penetrates your boat hull is a potential water leak. Through-hull fittings for live well intakes or drains, drain plugs, wash-down pump through hulls, screwing holes for depth-finder transducers and even the mounting bolts for your outboard motor could be a source for water leakage.
Recently I had a boat in the shop with a water-leak problem. As we were inspecting the hull, we found that the two bottom engine mounting bolts were loose. They had been loose for a long time. The holes in the transom were wallowed out, and to further complicate the problem, the bolts had not been properly sealed with marine silicone when they were initially installed.
We removed the engine from the transom, sealed all four bolt holes with marine silicone and reinstalled the engine. This one turned out to be an easy diagnosis and repair.
Another, sometimes more difficult to find, source of water leakage is the below-deck plumbing. Items such as hoses for livewell pumps and drains can become dry rotted and split, or as I found on a boat several years ago, the livewell hose can get chewed through by rats that have decided to make a nest deep inside the bilge.
Most livewell pumps are attached through transom fittings that are plastic. It is not uncommon for these pumps to crack and allow water to leak into the bilge. If this happens, you will need to remove and replace the through-hull pump. Any old sealant or silicone will need to be removed, and a fresh application of marine silicone should be used to seal the new pump you are installing.
Many larger boats are equipped with self-bailing scupper drains for the interior floor of the boat. If the drain fittings are damaged or the drain hoses are compromised, water may access the bilge. Scupper drains and live-well drains can be tested for leaks by running water through them with your garden hose and watching the bilge for leaks. If you see any leaks, you will need to replace the fittings and/or hoses as required.
If you have any questions about your boat, motor or trailer, contact me at email@example.com.