you are right on about some flourscent bulbs looking dim to the eye but still providing a bunch og energy to plants. GE makes two plant bulbs. the popular GE P&A bulb is usually a "wide spectrum" (it says so by the label on the bulb) version one that emits more visible light, but they have another "narrow spectrum" that looks really really dim. almost like a pinkish purple black-light.
one important thing to note though: while there is not a direct correlation between light intensity and photosynthesis (if the light is intense but at the wrong part of the spectrum), the more intense the light, the better it is at penetrating the water, so while the GE wide and narrow spectrum bulbs might grow plants equally well in air, the wide spectrum will be better at penetrating to the bottom of the tank