What focus mode and area are you set to, and in what shooting mode? You really should not have any problems acquiring focus in such conditions with the A550, even with slow lenses. However, the wrong modes can cause problems.
My suggestion would be to shoot in AF-S mode, rather than AF-A or AF-C. I'd likely shoot in A priority mode with the aperture dialed to full open, ISO on Auto or manually set as needed (likely ISO800 - 3200)...or alternately in P mode which might also work out well enough...I'd avoid any Auto modes personally. I'd set the camera to Spot focus, rather than center or wide/multi...that way you can point the focus spot at an area where you can acquire focus on the float (a person's face, a light, etc) and hold the shutter half-pressed while recomposing to take the shot.
If you're already using those settings and still having a problem focusing, I'd have to guess you have a lens and/or camera problem, as this should not be very challenging focus conditions for the A550 with those settings.