With Black Rose expansion there are eight paddle-wheelers and there are 16 passengers, so why couldn't you play this with 7 or 8?
You can play it that way. Here's a variant I posted in the file section to make it more fun:
Playing Mississippi Queen/Black Rose with 6-8 players can cause massive
jams which arguable hurt players moving last. To counter this, you might
want to try this variant:
During the setup, the idea is to use six tiles to create two separate
river branches that eventually merge together again. This allows boats
to split up at the start, making for a less congested opening.
1. Place the Starting River tile.
2. Connect two tiles to the Starting River tile -- one tile going left,
one going right -- to form two river branches.
3. On each of the two branches, place a tile going straight. (the whole
setup should look like a big "U" now).
4. Place at tile to re-join the branches. Overlap the tile's bottom two
hexes with the left and right branch.
5. Check to make sure the boats can easily get through the river. If
not, select a different re-joining tile.
Boats begin the race already in the water. Each player in turn places
and orients his boat in any river space on the Starting River tile. When
the game begins, players may drive down either branch. When boats reach
the re-joining tile, place new tiles as in the normal game.