missing polygons or rendering issue?

The general way to approach such a problem would be (note that with the web site design constantly changing, the process might differ slightly if you read this at a later time)

• go to www.openstreetmap org
• zoom very far into an area that has at least one line of the polygon you're interested in
• activate the data layer (click the layer button on the right then activate the "map data" checkbox)
• click on the blue line in the map and select "details" on the left hand side
• make a note of the way ID, and also look at the bottom of the node list where it should say "part of..." and point to a riverbank relation
• use pgsql (or pgadmin or similar) to connect to your database
• in the planet_osm_polygons table you should have an object with an osm_id of the negative (!) id of the relation
• if that isn't there, check if your planet_osm_rels has an entry with the positive (!) id of the relation and if planet_osm_ways has all the constituent ways; likely something is missing and therefore the polygon could not be constructed. Check your source data in that case. If however the building blocks are all there but the polygon is not, then for some reason osm2pgsql decided not to create the polygon - either due to a bug or due to tag filtering via the style file or LUA code.
• if however the polygon is there in your database but not on the tiles then there must be some kind of problem down the line, in your rendering rules.