You're buying a lens for beach shoots? Just curious... why not get one you can also use for non-beach shoots? Ha ha. Anyway I don't think there is a season. The manufacturers bring stuff out year round. When you see prices dropping dramatically or great rebates on an item, it usually means a better version is about to come out, but that's true in almost any business.
On David Mecey's recommendation a couple years ago, I bought two Tamron zooms (Nikon mounts). The 28-105mm f2.8, and the 70-210mm f2.8. Almost everything I've shot in the last two years has been with one of those two lenses and they are beautiful, and not really that expensive, one was about $700, the other about $900 if I remember correctly. Because I almost always meter manually, I refuse to buy a lens with a shifting f-stop. Once I get a digital camera I'll be testing to see how they work with that, but I don't anticipate any problems, especially since I don't shoot much wide angle.
Andy Pearlman Studio