I have seen on Youtube that they will sink if left in the water and enough rain collects.
Can confirm. It what ended being a funny story (it definitely wasn't funny as it was happening), my ex-girlfriend sent my 10' Porta-Bote to the bottom of the Dinner Key Marina dinghy dock just south of Miami.
We parked the dinghy at the dock early in the day to go on an all-day provisioning run. We were on our last day with a rental car and were loading up for several months of Bahamian cruising, so we were gone ALL DAY and well into the evening. We experienced biblical amounts of rainfall that day, and I was worried about the dinghy all day. We arrived back to the dock around midnight and quickly unloaded the car between raindrops. The agreement was that I would drive the car to a parking space while she carted the provisions to the dock (her choice, by the way, it was dark and she didn't want to walk alone at night). So, that's what we did.
When I made it back to the dock, I noticed from a distance that the dinghy was nearly filled to the gunwales with rainwater. What happened next occurred in slow motion... as I walked down the ramp to the dock, my ex walked over to the dinghy. In her head, she thought she could just lift up on the painter to dump the water out. In my head, I saw the transom loaded with my brand new Tohatsu outboard dipping below the waterline and filling the dinghy the rest of the way up with seawater. Just as I started to yell "NO!", she yanked up on the painter. Guess which one of us was correct about what would happen. I'll give you a hint; the dinghy promptly swamped and sank.
The rest of the story would take too long to type - perhaps I'll finish it over a beer one day. But, I'll give you this: it took at least 1.5 hours to refloat the dinghy and get the engine started. In the process, she sank someone else's rigid dinghy (that we also had to refloat), she fell completely in the water while a backpack full of cell phones a tablet computer and a handheld VHF, and everything we had just purchased was soaked to the bone with rain.
Thank goodness I got the engine started as the mothership was in the mooring field about a mile offshore. The valves must have just been closed on a compression cycle preventing the engine from filling up with water. I can't imagine how this story would have ended if I had to row her, myself, and all of our soaked provisions back to the boat (which took 3 trips), but it's likely our relationship would have ended a lot sooner than it ultimately did.
Anyway, the sun was coming up as I was rinsing and WD-40'ing every square centimeter of my brand new outboard.