Skater when if you figure out the tide stuff, post it here so Zorch, myself, and other DP junkies can use it.... please.
Good map, I would recommend some terrain other than the grass being raised, maybe some low spots? Sand tends to be smooth near water, and as you go up the shore you'll find buried kids, treasure, dog droppings, etc.
edit:
Using the func_model entity why not make a DP model buried waste down in the sand? That would look pretty sweet.
Or use Zorch's 'man in ice' idea and have a man encased in ice floating in the water to be used as cover... Much like you see cavemen in ice floating in movies.