As for snook fishing in SPI..........well we stay pretty tight lipped about them
As for launching a kayak....there are a few areas around the Sea Ranch Marina and Coast Guard Station area. These spots will be on your right hand side after entering SPI. There is also an area in Port Isabel (Mexiquita Flats).
A good suggestion would be to talk to Larry Haines @ The Fly Shop in Port Isabel. It is a small kayak/fly shop across the street from Pirates Landing & Marcellos restaurants. He can point you to a few areas.