There are several important considerations when placing beach lots:
-One side must be placed against the road
-One side must be placed in water
-No more than half of the lot can be in water
According to the Wiki, when placing a pre-built beach lot, the slope from the road down to the beach that was used in the pre-built lot must also match the slope from the road to the beach where you are trying to place the lot. There is a tutorial that explains how to match the terrain and pre-built lot (and how to build beach lots using the terrain editor from Free Time) at
http://www.modthesims.info/showthread.php?t=278903
