I found a couple really nice terrains for beach lots at mts2 by CardiganAnjel and by Caupi
I've recently looked at/checked in game
many terrains to see how they handle beach lots
I didn't check the Diamondbar Islands/Delta terrains by Caupi but they look like the beach lots will be nearly flat (which is wonderful).
the run down for the ones I did check
by CardiganAnjelEchos of Simology - really nice and nearly flat beach lots, with a bit of fiddling you can fit quite a few beach lots. I suggest if you get this one that you place beach lots first, then if you screw up the terrain, you can just start over and not make the same mistakes (thats what I had to do, currently holding onto that hood for future use)
Miho Island 2 - really nice (if you want to see pictures click my Despair link and look in my "Despair and The Story" thread), you can fit exactly 25 (of the smallest), nearly flat, beach lots. Two of my pictures are of my new Public Beach community lot and in other pictures you can see exactly where the 25 beach lots will fit.
Miho Island 3 - really nice (almost used this one instead), with a bit of fiddling you can fit more then 25 (smallest) beach lots but they won't all be nearly flat, some will have more of a slope from the sidewalk to the beach.
Avoid "Sand Simolean Point" by Phaenoh, it's sad to say that. That terrain is really nice and I was considering using it but it SUCKS for beach lots...unless you like needing stairs to reach the beach. it's so bad the slope starts at the sidewalk
some other nice ones can be found at
http://www.simsanne.com/Forum/filebase.php Insel-19 - I was using it for Despair, the slope on the beach lots wasn't bad to play but sucked for taking pictures. you can see pictures in some of my other threads via my Despair link.
I haven't checked the other ones there except for Land1, some look like the beach lots will be nearly flat. They were nice on
Land1, briefly considered using it for my new hood but I disliked the mountain (to close to the roads) and it wouldn't have been big enough for all my sims, specially when trying to stay away from the mountain/hills.