A peice of 1/8 to 1/4" plywood or similar makes a perfect launch pad, even serves double duty as a handy screed to get the sand level before you place it.
As has been said, had catch is the best practice over sand.
How high you fly is your decision based on what you have in mind. Just remember the barometer can be off so allow a safety margin. I don't go lower than 10m unless I am close enough to judge height comfortably.
Good thing about flying close to the ocean is provided the horizon is in sight you can use it to judge height of potential obstacles, if they appear lower than the horizon you have a great chance of clearing them at your current altitude.
Pay close attention to other AC movements. Yesterday I saw to AC (police helicopter and shark patrol) both well below 400ft, if I said they were at 200 that would be a generous estimation.
Given people at the beaches tend to be wearing minimal clothing don't fly close even to go over, they might suspect you are pervimg, most don't realise how small they appear in the frames with the wide angle of view. The exception might be the one Sheila who suggested I was filming her (you could have seen her from the moon).
