code: public boolean isWaterTile(TileCoordinate tc) {
          List<TileCoordinate> tiles = tc.translateToZoomLevel(TILE_INFO_ZOOMLEVEL);
          for (TileCoordinate tile : tiles) {
              if (!this.seaTileInfo.get(tile.getY() * 4096 + tile.getX())) {
                  return false;
              }
          }
          return true;
      }
docs: Checks if a tile is completely covered by water. <b>Important notice:</b> The method may produce false negatives
  on higher zoom levels than 12.
  
  @param tc tile given as TileCoordinate
  @return true if the tile is completely covered by water, false if the associated tile(s) on zoom level 12 is(are)
  not completely covered by water.