Motel: the guy was the motel owner and when he got there he found out the place had gone up in flames?
(sorry for bringing this back but I didn't notice the answer to this one yet ;))

lake: you measure the salt concentration of the lake. Pour a few bags of salt in the lake and stirr it well, so the salt concentration is more or less the same in all the lake. Measure the salinity again. From the concentation and knowing the volume of salt you poured in the lake, the approximate volume of the lake should become obvious.
But who cares about how much water there is in the lake anyway? ;)