Is it okay to put a heat pump water heater in a crawl space?
You may be able to:


Height - The website says the heat pump water heater (HPWH) is 60.5 inches tall. The hot and cold pipes enter at the top of the unit, but it sounds like 7 ft is enough vertical space.
Volume - I'm not sure what the overall volume of your crawlspace is, but the GeoSpring installation Guide says:



    With the installation of a louvered door, it can be installed in rooms smaller than 10' x 10' x 7' (700 cu.ft.). Louvers should be 240 square inches (0.15 m^2) or greater.


So, if your crawlspace is vented to the outside, or as HerrBag states, is conditioned, with air exchange to/from the house, it will be fine, but may operate at lower efficiency:


Temperature - Like you say, the water heater is a heat pump or "reverse AC", meaning it heats water by pumping heat from the air. In other words, it cools down the space it is in. If your crawlspace temperature gets below 45F because of the heat pump or just because it is cold outside, the unit will operate at a much lower efficiency. This study states that HPWHs take 10-15% efficiency penalty when installed in confined spaces. 
Humidity - High humidity is not a problem. In fact, it will operate more efficiently at higher humidity. Freezing temperatures are an issue for efficiency, as stated above, as well as the obvious danger to your pipes. As HerrBag stated, a floor drain or pump/sump will be necessary.


What climate are you in? If you are in a hot climate, it is advantageous to put the heat pump water heater in your living space to take advantage of the free cooling.