How can I provide natural light under a roof?

I'm building a platform out in the water, sort of like an oil rig, and I'm realizing that the area beneath my platform is going to become completely dark at all times. In order to prevent that, how can I let beams of sunlight through? I'd prefer not to use glass, or to just make arbitrary holes in the platform.

I've seen some good answers on this question so far, but there is a way to to light the underside of the platform without making holes that go all the way through by using artificial means like lava.

If the platform is at least 3 blocks high, you can hollow out the middle layer and fill it with lava, with a pattern of glass below like so:

XXXXXXXXXXXXXXX  X = Stone (or whatever)
XLLLLLLLLLLLLLX  L = Lava
X^XXX^XXX^XXX^X  ^ = Glass

Perhaps you could pattern the glass on the bottom layer to resemble flourescent lighting. If your building material is flammable, you can replace the lava with lightstone or torches.

If your only goal is to let "natural light" Sunlight / Glass are your only options. If, however, you'd be happy with something with the same Luminosity as natural light, you have a couple options.

Fire, and Magma (still or flows) give off a luminance of 15, the same value of natural light, and magma in particular, due to its flow, can light a wide area. However, both of these are dangerous if walked into.

Glowstone and Jack-o-lanterns both give 15 luminance as well (compare this to a torch's 14; due to the exponential brightness programmed into the game, the difference is actually pretty noticeable. A Torch gives only 80% of the brightess as luminosity 15 sources do.) are both safe to walk on / bump into and function perfectly well underwater (Torches, magma, etc. are destroyed on contact with water).

A final option would be to simply alter the texture (or download a texture pack) of the glass block, so that it appears as something else (but still allows light to pass through).

An interesting alternative to glass would be trapdoors. I know this still basically means poking holes into your platform, but it gives a less jarring and "hole-like" feel than just a glass block because it's not entirely transparent and still flush with the floor, while being perfectly transparent for lighting purposes.

With a wooden platform, a wooden trapdoor fuses much more naturally with the rest of the floor than a hole of glass and especially in more "industrial" builds (holding onto your oil rig example) the iron trapdoors add a great deal to the "technical" design and fit especially well into a stone slab floor.

I know this question is a little old, so you might not have considered this option back then, since trapdoors got quite some overhauls making their usage for design purposes a little more flexible over time beyond their mere door qualities.