RE: FE BOARD + Drake 500
If you aren't racing, and when using such a small sail (8.5 m2) the 50 cm fin (500 mm) should work pretty well.
You won't get quite as high upwind, but you should be faster on a reach or downwind.
Give it a try.
Also, for shallow conditions, think about a weed fin.
You won't get quite the upwind performance, but you get the added safety because the angled back leading edge will "lift" when you run the fin aground, and you don't get tossed of the front.
If the 500 mm seems a little small, there's lots of sizes from the stock 700 mm down to the 500 mm. Look for a fin that suits the depth of your shallows with just a little clearance.
Hope this helps,