Boy, I can write a book on this topic!!! I am 42y/o and have been involved around boats since I was a little kid, making my first bluewater trip when I was in the 4th grade. Unfortuneately, I have had the chance to see some things over the years that just make me scratch my head.
An old friend of mine once had a Redfin center console boat that went back to the factory for some interior glass work. When it came back, he ask me to come run it with him since the boat had been sitting up. Well, after about 30 minutes of running, we stopped the boat and it would not get back on step. That's when I noticed water coming out of a hole in the floor of the boat. I checked the bildge to see it completely full of water. We hit the pump, and I jumped in the water and pulled the plug. He thought I was nuts and just knew he was going to sink. We kept the engine running and made it back to the launch. Once on the trailer, we had to let the boat empty since it looked like his single axle trailer was going to break and axle. It took us almost 2 hours to get the boat secured. Investigating it we found that the hose leading forward to the front livewell drain was not connected at all, so it was just flooding the boat.
I have seen more problems with hoses and tubing under the floor of boats than I care to talk about. And no matter the make or model, if they have plumbing under the floor, they are at risk of having something happen! I fished with my attorney for quite some time out of his Triton Sea Flight 22'. One day he calls me on the radio and said "please come help me, my boat is sinking". Well, we scream over to where he was fishing about 10 miles away and sure enough, the water is coming over the transom. I hooked on to his bow, got him and his family on board with us and drug him for what seemed like forever. By dragging him, I was able to get most of the water and weight to the back of the boat and roll out over his already low transom. The pump was able to keep it out long enough for us to get him to sand bar. We found a plastic "T" which connected 3 - 5/8" poly hoses had broken and had flooded his bildge. This boat was not rigged w/ a isolating valve like Blazer Bay boats, so I had to rig a plug and seal the hole from the outside. (smashing fishing corks together with your foot will close a hole quickly)
Last year I was part of a 3 man and one woman (my wife) crew for a maiden voyage of a 31' Contender in the Gulf of Mexico. This is a $250K rig and yep you guessed it, on the first trip offshore in flat calm seas, we had problems with the plumbing. This boat was equipped with an isolation valve and is was not a problem, but I found it early and the owner thought I was nuts for making him stop after running for 20 minutes for me to check the bildge. He could not believe his new boat would have this problem, and more amazed I wanted to look for it.
As you can see, I had several incidents with plumbing in the bottom of boats. As I write this, I am taking a break from rigging up my own 2400 that has yet to hit the water. Besides all the elctronics and gadjets I installed, I took the time to go through the bildge, which was great timing since the motor was not on it and made access somewhat easier. I found that my own boat had not one, but two of the very same plastic "T's" that I found broken on my buddy's boat a few years back. I took pictures of these, removed them, and replaced it with a brass set up. It cost me about $45 dollars to make the conversion, but I have the piece of mind knowing that 5 years from now, they won't break during a trip!
Anytime someone tells me they are having water enter their hull and can't find it, I immediately ask if they have a valve in the boat to close their intake to the livewell pumps and washdown. If they do, I tell them to take the boat out with the valve closed and if no water enters the boat in that scenario, then the problem is in the plumbing. If they do not have a valve to close it, I tell them to get a plug and plug it from the outside while on the trailer. Hope this helps and I will post pictures of my rig out soon so everyone can see what I did in my boat.