47F is 8 C. Based on the warning at our local authority run lake, below 10C (50F) I'd say you really need wetsuits.
The problem is not keeping warm on the water but rather what happens if you go in. Hypothermia can set in quickly so I'd be cautious especially as with a boat you're exposed to wind across the tramp, both actual and apparent when you come back out and this speeds up the chilling factor massively as the water evaporates.
BTW if you get wetsuits get good ones. You really get what you pay for both in terms of flexibility and also sealing qualities.
For surfing I use a Rhino Venom front zip which is the dogs.http://shop.rhinosurf.com/rhino-mens-ve ... 2-97-p.asp
The Revolution zipless is supposed to be better still.
Both are very similar to the Excel Drylock ranges but a fraction of the cost. Effectively they are summer steamers and seal very tightly around all areas so don't flush with water when you go in. They are also very warm.
I previously had a RipCurl 3:2 wetsuit and it wasn't a patch on the Rhino for warmth or flexibility. The last time I used it before changing I was freezing. I've never even felt cold in the Venom or even noticed the cold water, its toasty.
I'm unsure how suitable they are for standing up to the rigors of sailing though. I'm sure others can advise you.
Flexibility is 2nd to no others I've found which is how you can get your entire body through a 10 inch zip!
The Revolution is so flexible it doesn't even need the zip!