Yep, water is a great way of storing heat as it can store a lot of heat per volume. See the wiki
here for info on thermal mass.
I'm guessing it would be a pretty large pain in the butt to cut an access hole into the crawlspace. My basement walls are probably 1.5-2 feet thick and is a fieldstone basement (stone + mortar). I believe I'd need a jackhammer to get through the wall. If anyone has any ideas I'm all ears.