Congratulations, Vonne, on completing a novel. That's no mean feat.
Of course, writing a good synopsis can be as challenging as weaving together the full scope of 80-odd thousand words.
Now, I'll admit this isn't a genre I would usually look at, so I can't comment on the story itself - though you have clearly put good thought into that side of things. (There are clues in your synopsis that there is a good story there.)
What i will comment on is the structure you've used.
You open with scene-setting. The first 28 words are pure scenery. They are an attempt to be scinematic. (Aside: this suggests you've probably done the same in the manuscript, not getting to the action fast enough.) Then, after the shark attack, you spend almost 150 words giving Josh's backstory. It is, in effect, fluffy.
Here's my take on rewriting the first 200 of your synopsis (obviously not your voice):
Prodigal scion of an Australian industrialist family, JOSH BRENNER embraces professional surfing as his vehicle for success and personal validation. In San Diego to compete, he suffers a brutal - career-crippling - shark attack. DR. MIA DE LA CRUZ, a young trauma surgeon at a nearby hospital, is first on the scene, administering first aid until an ambulance arrives, and later seeing Josh in her professional capacity.
That's 65 words, leaving more space to develop the struggles of your story. Also, you don't need to aim to hit 500 words; you should only think of it as an upper limit. Less, as they say, is more.