I'm partial to shorter lenses -- I have everything up to a 600 f4 but find the longest I need for a beach shoot is 200mm. The downsides of bigger lens, to me, outweigh the pluses. First, communication: Anything over a 200mm I find myself having to yell (usually over wind and waves) directions and the model looking at me with a confused expression and yelling back "what, can't hear you..." -- this wastes a lot of time and energy. Also, weight and size of anything over 200mm generally requires a monopod or tripod to do decent work, which is fine but you lose mobility and end up shooting many of the same angles, not just because of the 'pod but also because bigger lenses are harder to move and require long distance between lens and subject, making them less versatile -- not to mention the backache from lugging a big lens around in the sun. As for blur, sand and water fade easily into the background so no need for tremendous blur, and many times you don't want too much blur if the background is nice. Finally, many of my favorite beach shots have been taken with shorter lenses -- even 35mm -- I just like the perspective.
In sum, probably the best lens is the one you are considering or something else, as Andy Pearlman suggests, in the 70-200mm zoom range -- many pro swimwear shooters favor this type of lens lens for it's versatility and quality. Have fun, shooting on the beach is the best. [img]/ubbthreads/images/graemlins/grin.gif[/img]