True the do grow in shallows but they also grow in waters much deeper, it all depends on how far the lights are from the top of the DSB. the way I suggested would be the cheapest but may not be the most efficient as far as total par. If you have VHO ballast that would be even better like an Icecap 660 which can run 4x 110w bulbs would be even better and cheaper as far as a new T-5 set up.

You could also check out Hellolights.com and get one of their retrofits in a 4 bulb for around $200. I got the T-5 2 bulb retrofit from them and it is quality. If you have a electrical supply company in your town you can even go buy a ballast from them for even cheaper then wire it yourself and buy bulbs....I did that on the last tank I had. Depends on how much you want to spend, I usually go with the idea of get a bit more then you need this way you don't have to spend money to upgrade at a later time because what you got in the first place wasn't able to do what you wanted.....
Here is a great article on sea-grass:
http://reefkeeping.com/issues/2006-04/sl/index.php