Hi,
There are a few things to look for...first, where do the people hang out? Where do they play in the water? Second, what is the beach geology like...are there deeper regions parallel to shore, does it have any ravines, what is the seasonal sand deposition/excavation like? Basically, look where the people are, then see if there are any places more likely to be hiding any goodies. In my area, sand is deposited in the summer and removed in the winter. So, the summer is good because of more people, but the winter can be good because some sand is being removed from the beach.
Another aspect is difficulty. Searching the wet sand or shallow water is pretty easy and you can cover a lot of ground whereas searching in the surf (depending on wave size/action) can be a demanding task. Most casual detectorists don't venture into the deeper water, however, so I tend to water hunt on beaches that have been hunted a lot. I do search the wet sand and shallow water too in order to get a feel for the beach.
The weather, wind and tides also play a large role. If the tide is high, or there is a stiff offshore breeze blowing in, the area you want to hunt may be too deep or too rough. Best bet here is to use a tide calculator on the web and check the weather. If you get a low tide and have a wind blowing from shore to sea, you can sometimes go out really far and hunt without much difficulty...I live for those types of days.
Hope this helps, take care and HH!