Probably. One may as well go all the way
though.

Plants need three things to grow:
water/minerals, CO2/O2, sunlight.

These three things are abundant on the
surface of the oceans, therefore GM plants of
the "water-hyacinth" variety might be
convinced to grow while floating on the
surface.

Some plants such as kelp (which have small
air-filled bladders to keep them buoyant)
have little trouble with seawater, and any
floating variety would be easy to harvest and
process; like whales!

Compare this with Canola which is a GM
rapeseed plant, and the ease at which it
grows in vast flat areas and how easy it is
to harvest with mechanical devices.

Large masses of floating weeds would have a
calming effect on the ocean surface (like
"oil on troubled waters").

The Sargasso sea (calmer than most) might be
a good place to start.

P.S.

try
http://en.wikipedia.org/wiki/Sargassum

for a ready-made biomass.