Basically, similar to a trap door/regular door, but with water flow. Grates of some kind that you can place on a floor that allow water to flow normally as though nothing is there, but the player can stand on. There could be two variants to this, one that allows items to fall through and one that doesn't. Maybe a "wide" filter type of grate, with large holes for aesthetics, that items can flow through, and a smaller type grate with smaller holes that items cannot fall through. The aesthetics could just be so that players know what style they are using. Also, a vertical style grate that allows water to flow normally and that items can travel through, but players cannot walk through.
There could be several practical uses for this, such as waterways that carry items to a destination, and for waterfalls that you can travel through, then land on a grate that lets the water keep flowing down so you don't flood everything in sight.
Already a floodgate mod, a solid block that lets lava/water falls through. There is a piston mod that acts like a floodgate and much more.
if you hate mods, then wait for pistons to be released into the actual game. probaly beta 1.7/8