I fixed some of the bugs near New Orlean, check osmarender zoomlevels >= 12

I agree, it need not be closed. But sequence of ways must form closed shape. Sand Cay consisted of one (unclosed) way. It's error. After it was fixed, osmarender rendered tile fine.

Zoom levels < 12 will be updated later.

Hmm.. comments do not like < sign

opps...
rest of the message:
Zoom levels <12 should be updated later.

Now everything should be ok. I fixed final errors (there was leftover segment(two nodes) and Sand Cay was not closed).
http://openstreetmap.org/?lat=19.3081&lon=-81.2716&zoom=12&layers=0B0FTF
Zoom levels <12 will be updated later.

If segments are combined, there are smaller chance for error. If resulting segment have too much nodes, it can be split again.

Now I really merged coastline together (there where two segments). Also changed direction of small island to be anti-clockwise. Hope this helps.