The best resume of the Waterboys is on Ceolas, athttp://www.ceolas.org/artists/Waterboys.html
The greatest hits is the best purchase.
You might want to check out World Party, as Karl Wallinger (frontman) was in the Waterboys, and writes stuff in a similar vein ('Put the message in the box' is, IMO their best riff).
Enjoy
Skipjack