There is a setting to turn off aquifers, and aquifer indications have occasionally been wrong. An aquifer has always been found in the soil levels for me (sometimes extending down into suitable stone), but the top has always been shallow (probably never deeper than 5 levels below the surface).
If you're using DFHack you can follow up on Luriant's suggestion by using the tool script here
http://www.bay12forums.com/smf/index.php?topic=160856.0 which can show you where your biomes are.
However, digging 150 levels to reach cavern 1 sounds like a mountain, and that's not a likely location for an aquifer. Thus, I suspect you need to secure cavern water. That can usually be done reasonably safely by digging out a cistern in the rock beside a cavern lake (preferably one with an edge access so the water won't run out), build a drawbridge airlock in a tunnel up to, but not through, the wall to the water, dig a tunnel above that one with a drawbridge, hook all the drawbridges up to levers (and test that the hookups are correct and the drawbridges are raising), and then send a miner to channel down on the wall separating the water from the tunnel below (and air/rock from the tunnel above) when the cavern seems reasonably safe. Close the upper drawbridge after the miner has retreated. Brick up the access to the cistern while it's filling, close the airlock drawbridges when done and remove the blocking wall, and then top up the cistern periodically by letting water into the airlock, close the outer bridge, and then open the inner one.
Don't try fortifications, as submerged ones can be swum through.