If you are interested I would have a look at some of the good dinghy supply shops - such as
http://www.piplers.co.uk/category.ph...=142&catpath=9
just chosen at radomn, not to advertise them in particular.
I can vouch for dry suits such as these being a whole different ball game. they are designed to be flexible and comfortable to wear - moving around in a high perfoamnce dinghy is nothing like you would imagine if you havent sailed before or have sailed yachts.
Whether they ultimately provide the same insulation as an immersion suit I dont know but I would imagine they are very close as ultimately the suit works because of its ability to keep you totally dry not to insulate you - the insulation comes from the furry bear suit underneath!
However you look at it four dry suits are not going ot be cheap - if you are going to provide for a full load of pax.
Personally I always carry a raft. Again they are not cheap, but these days they are really small and compact - you can probably avoid servicing them every year and even then the cost is only around £60. I have just had mine back from SEMS.
I think a raft is a good half way house - it might not be the perfect solution, but its better than a jackets alone - and I suspect you can always sell them on or rent them to others if you wish. there are also a few firms around that will rent you rafts for one off trips.