It's the light - evening, I think...or early morning - and that gloriously clear water.
Over in the thread which retow started about "Reactions to Photographs"...so many responses have brought up the fact that pictures speak to us all individually and often tap into our private memories. For me this image of yours brings back a very happy childhood full of summers spent swimming in and poking around in the saltwater... I think I may have been an otter in a former life. So, Sue, your picture brings a lot with it to my eyes. Many thanks - and I urge folks to click on it to see it in its larger size.