[OSM-talk] coastline within park

[Robin Paulson]

i've got a situation come up, that i'm not sure how to map:

http://openstreetmap.org/?lat=-36.9241&lon=174.77611&zoom=17&layers=0B0FF

this shows a park which is near the coast. it includes an area of
water linked, by a man-made tunnel, to the ocean, wholly contained
with in the park. previously, it was completely open to the sea - the
motorway to the south since built on a causeway has cut it off, apart
from the tunnel. therefore, the area of water is tidal, and hence is
demarcated from land by the coastline

how could this be mapped/tagged? initially, i tagged the coastline as
layer=1 to make it show up on the renderers above the park (by
default, park renders above coastline AFAICS).

any ideas? should the lagoon be tagged as coastline as i have done, or
is that stretching a point too far? i don't think the park stops at
the water's edge, so cutting a hole in it is not suitable

this also brings up the question of what happens when a park (such as
the great barrier reef) extends over an ocean/islands: does it obscure
what is underneath? is there a way round this?