Most lowboy gas water heaters are 4 feet tall without the chimney. They can not be laid on their side. They still require proper venting and placing them on the ground is not permissible, so they would require a venting kit or a power-vent. The venting would also have to occur over the snow line and away from openings by code.
Installing a water heater in a crawl space is just a bad idea. Who wants to service one under the house? Who wants to worry about crawling things and pilot or electronic lights? The unit would have to be set on a slab under the house - just as well to build a small partial basement room there.
The obvious reason for not going with an electric heater is the average cost being $300 per year for a typical family with gas and $800 per year for an electric heater. Insist on the builder making a little alcove or louvered closet for your water heater. It is so easy to install a tall direct-vent water heater in a small space.