There are a number of reasons not to use a RIB
(Dinghy) as a Liferaft. Most of them have been pointed out in the previous posts.
A dinghy is designed to take you from point A to B, not bob around in the ocean for extended periods. Most have low top sides and without a motor
running will swamp in relatively moderate seas. Most dinghies have no sun protection, so in the tropics it would only be hours before you were suffering from heat exsposure or in the colder climates, hypothermia.
People make mistakes purchasing
liferafts.... They tend to lump them all together and most times use price
as the deciding factor.
Liferaft construction is very different, based on brand and models. Some liferafts wouldn't hold up in a backyard pool, but they were inexpensive so the owners were happy.
Your primary consideration should be where are you sailing and how many people could be potentially using the craft. With that in mind you have to look at the raft construction, size and of course what goes inside it (provisions). If you wear reading glasses, take special meds or anything else you require, should be packed into the raft.
You also have to consider how you are going to deploy the liferaft. Some people are happy with valises, but I all to often hear someone asking for help, when they need to get it out of the storage
locker. You should be able to be deploye the raft with one average person. I say this because all to often people forget you may not be the one deplying the raft, because your injured or doing damage control or figuring things out.
The hard cases they will slide across the deck, making lifting unnecessary and will automatically deploy if you can't get to it in time. These are my favorite for that reason.
After taking my first STCW class, I realised that it is very difficult to in the raft from the water
, especially in wet clothes. Most rafts come with nylon "rope ladders", which are worthless, especially in rolling seas. A boarding platform is better and easier to get inside your raft.
Occupancy is the next thing to consider... I think the industry standard is about two square feet per person. In the real world, when I was in STCW, we got 14 adult males in a 20 person liferaft and tried to figure were we would put anyone else.
Lastly, think about abandoning you vessel and what you would do... Vessels sink a varing rates depending on damages and sea conditions, among other factors. You could have hours or only minutes to get off the boat.
In regards to water proof bags, pre-tie lanyards to them and attatch them to a warm body. A friend of mine lost
a boat to a whale strike a couple years ago. As they were abandoning ship a large wave swept the cockpit
and took their ditch bags with it (Inmportant documents, money
, passports and spare food/water).
So with all that rambling and IMHO, unless your sailing on small lakes or in a place were rescue
in less than 15 minutes away, I would not suggest a dinghy be used as a liferaft.