As a San Diego resident I know what we want and I know what they'll give. The Coastal Commission is rather stringent for reasons that don't sound as professional as they do personal. Take for example the hand-written comments on page 2 of this document from 2003 by the Coastal Commission. ( http://documents.coastal.ca.gov/reports/2008/4/Th4e-4-2008.pdf ) For those who don't want to click it says "Enough already, just say no to Budweiser." All they see is beer and trouble, they miss the fact that taxpaying voters want it and will bring income to the region. The decisions feel akin to an HOA run by a board of retirees.
San Diego only has one major waterslide park ( Knotts ) and I don't mind saying it's pretty uninteresting, not to mention well off the beaten path. The few municipal ones owned/operated out here seem to have the theme of "grey concrete" which is cool if your waterslide park's theme is East Germany.
Mission Bay could be an epic tourist attraction for San Diego, but I suspect that the powers-that-be in San Diego would rather reserve it as a yacht club for themselves and their rich buddies, and continue making life difficult for we riff-raff theme park fans.