We hunt open water boat blinds for gadwalls, occasional wigeon and pintails & divers on Mobile Bay. It works well unless we don't get new birds often, then the gaddies get really spooky. Some advice:
Keep the blind as low as possible. The lower your profile the better. A lot of commercial blinds are too high my opinion.
Try to taper the ends to avoid looking like a box. Not many boxes in nature.
Cut some cane or other native vegetation like willows (we use phragmites & bamboo) to stick in the bottom around the boat to break up your outline even more. This will help hold the boat in place as well.
Last, build your own blind & save a pile of money. Why spend $400-600.00 on something you can make yourself for $100.00?
Here is my blind, a simple pull up design that uses cerex. Under a $100.00 in the whole thing. This picture is from when I first built it. I cut it down a little more. With an 18' boat, you will probably need a middle support. When I I trailer, I simply drop it off the support poles and use 3 small bungees per side to strap it down to the seats. Wish I had some better pics.
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