Laser levels are cheap but you have to get them set level before they are accurate.. Just like a transit, you have to level it with a bubble..
You also need something steady to rest the lazer level in while your pivet it to aim at each corner or reference point. Each time you pivet it you need to check the level bubble..
So a cheap laser may not be any easier or accurate than a water level..
I have always use a long clear plastic tube filled with water, however the last time I need this, I found a kit for use on a garden hose.. It's just two 1' long plastic tubes that screw onto the ends of a garden hose..The only problem here is you can't see if you have bunch of air trapped in the middle of the hose..
When I had my mobile (mfg) home set up, they used a clear tube but they had a 5 gal tank on a stand and the hose came off the bottom of that. Plenty of volume so there was never any concern about loss of water.. The tank was set up at a height for convenient reference and never moved for the whole operation. Hose was long enough to reach all points under the house. Only took one man, where a plain hose always required a second person to make sure his end was on the basic reference point.
Water line levels are still used in most of the third world countries where transits are few and far between.. An engineer or survyor may locate a reference to start a building, even a multi story and the on site people will use the water line level to measure from the reference.