Generally to my understanding while most likely just a ruse an emotional support animal is not a pet but a medical device. 

You could be showing a house and the Sellers can listen to you via a laptop, a security camera, their Alexa, or other spy devices. 

Check it's nightly rate (NOT what is listed, that is a misleading marketing device), to check the proper nightly rate just find some dates (in the future a month or two, nearby dates have discounts) and do an entire week.

., Apple) devices, so you can't use it to call or text just anyone

Is there some device I could buy that separately meters the water usage?

Salt air rusts out all mechanical devices like ac units before their time.  

(not a big deal, easily fixable)25) No Arc Fault Breakers in required areas of electrical panel26) Doorbell does not work (not a big deal, easily fixable)27) Branch circuits miswired to a single bus bar28) Gas furnace does not have a sediment catcher29) HVAC does not have overflow drain pan30) Water heater is a 2009 model (was "replaced")31) Corrosion observed on the hot water handle for laundry connections32) Toilet loose in bathroom (bathroom was "rehabbed")33) Gap between wall and faucet in guest bathroom shower (bathroom was "rehabbed")34) Hot water handle in master shower spins and does not turn on (bathroom was "rehabbed")35) Leaks under all sinks observed36) Dishwasher drain hose did not form a loop and create a trap under sink37) Door on dishwasher does not latch 38) Disposal was not plugged in and was not tested (probably doesn't work)39) No anti-tipping device on stove, and back left burner does not work40) Sprinkler system was not tested.  

Just link the device to one of the tenants’s internet.

Do you think this device is ever appropriate for buy and hold short term rental portfolios?

Marketing is all about knowing the painpoint of your audience AND intercepting that painpoint through a message that inspires relief and confidence you can help them.This is why I would device a SPECIFIC message per specific audience so you really speak to a specific audience.