Not currently.
However, the new release WILL allow you to build stairs down from the top, so you can pump out a hole in the water, build stairs down, wall it up, and try to make a dwarven
cassion to work from.
Heck, there's going to be a whole bunch of people trying out the sea fortress idea again next release. It will be interesting to see how people solve the problem of going more one z-level down without the use of magma.
I've heard rumors that people have managed to get to the bottom by the pump ring alone, but I really don't see how it's possible...
Wait, I actually do have an idea.
Not just a ring of pumps, but a ring of pump towers. At water level, you pump IN. One floor above, you pump OUT. With all the pumps working simultaneously, you should then be able to construct the next pump level (out), which will be picked up by the ring that moves the water in, which is picked up by the top level, which moves it out of the cassion.
I will have to try this. Hmm...
Actually, a pump and floor dome structure would be even better, as you could have more draining in a larger area and build the power mechanisms from the top down, and even have the waterwheels up on top. I think.
But right now, implementing it would be impossible because you can't build down from above or sink anything without using a magma cast.
And now I'm wondering if you can build a double-hulled magma cast that would allow the construction of cassions and pylons all the way to the bottom.