I think my favorite Waterboys album is "Room to Roam", but that is also the most 'folksy' one, for want of a better word.
"Fishermans blues" and "This is the sea" are safe bets, but I would suggest "Live Adventures" which is recordings from shows in Ireland, Scotland, England and Holland. Not only is it a really good mix of the Waterboys over the years, there's also some interesting covers thrown in...Prince's Purple rain, Dylans Death is not the end and Springsteen/Patti Smiths Because the night.