Take a look at the wind speed/power curve and compare it to your local wind speeds. That 400W is at 25 knots of wind. At 15 knots sustained (which you will not see very often in a marina), you get maybe 100 Watts out of it.
And note that once you get over 25 knots, the output again falls away very quickly to almost nothing.
The manual has a very deceptive graph - I mistrust any outfit that has to fudge that sort of thing.
I also note that West marine
has 1 x 5 star rating and 4 x 1 star ratings! The only 5 star rating is by "Staff Reviewer".