Here is a link to the system I bought. The same seller offers different versions (e.g. some wireless, some with 4 to 8 cameras, think RV or horse trailer use, a camera for every horse). I mounted one of my cameras to the back of the tow vehicle to make hitching the trailer to it easier. The other is on the boat trailer, its designed to be on continuously so it doesn't overheat even when I leave it on for over three hours at a time. If you go this route make sure you get the water resistant camera. I can toggle the view to each camera on the fly. In order to use the same display to view people being towed behind the boat, you would either have to go with the wireless model or buy two separate systems, one for the truck and/or trailer and another for the boat.
The system Raphael cited is probably fine for its intended purpose as rear view only while reversing camera but I'd bet it will overheat or otherwise malfunction if left on for too long, that's what at least two other similar systems did when I tried to keep them on for over 10 to 15 minutes.
Two things to consider if you buy what I bought and do what I do with it.
1. The camera on the boat trailer is all but useless at night, all I see are head lights which tend to overcome the entire display (this true no matter what display mode I choose (e.g. vivid, natural, etc.)).
2. At night, I usually switch to the camera mounted to the tow vehicle, I still get some view of who is driving next to either side of the boat and the hitch/connection as well.
http://www.ebay.com/itm/290514309867?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649
Lastly, here are a few pics of the camera views that I took with my iPhone when I went boating recently (in the last month or so).
View attachment 2907 View attachment 2908