I just did some quick research and it looks like parking at BB/TL is free to all guests, even those who are staying offsite. I'm not sure it is intended for people to leave their cars there while visiting other areas of WDW though. But in theory, you could go to a water park in the morning, take a bus to an EPCOT area resort, get to EPCOT from that resort, and retrace those steps back to Blizzard Beach to get your car. That seems like a load though - especially if you're staying at EPCOT until late when the water parks will have been closed for hours and the buses stopped running there.
If you pay for parking at EPCOT, it's $17. Parking at DTD is free as well. I would just drive from place to place seeing as 2/3 of them offer free parking. The time and hassle you save is worth that.
That's the most help I can offer you - sorry!