Wfter the first reshuffle, go down to drawing 1 card per turn. Since you'll have all the cards drawn fairly quickly, the challenge becomes maneuvering them to the right player. Usually by the time you get through the entire deck this way you've gotten at least one treasure and discarded the 4 cards, which means you won't have every single card be a "Waters Rise!". This should let you play with 5 people just fine, and possibly 6 if you start at novice level.
If you are willing to do a little bit more work, you can beef up the game itself. This will give results closer to the feel of the original game. Keep using the rule I mentioned above! Here's what you'll need:
- 1 new treasure and 5 treasure cards for it.
- 6 extra tiles and their flood cards. 4 of them are just normal tiles. 2 of them correspond to the 5th treasure.
You can use index cards to make the tiles and regular playing cards for the flood and treasure cards. The four aces are the flood cards for the 4 new normal tiles, the King of Spades and King of Hearts are the flood cards for the 2 5th treasure tiles. 2-6 of Hearts are the treasure cards for the 5th treasure. Yes, it does look a little shoddy to have regular playing cards mixed in with the deck, but it works just fine and I've found it rarely hurts the gaming experience. If this still bothers you, just sleeve all the cards.This will let you handle 5 people just fine.
To play with 6 people, add one more card of each of the 5 colors, so there will be 6 cards of each color. I just used a marker to color some normal playing cards to match the others. Since you now have added cards for the new 5th treasure and the old treasures, the issue of knowing exactly what's coming up by what the card back looks like is reduced. I also recommend that you allow players to either give or take a treasure card from another player on the same tile for an action, instead of allowing giving only. This lessens the time spent waiting to get to the right player's turn. I have played this 6 player variant several times with great success, to the point where I actually prefer to play with 6 people to playing with less.
If you want to play with 7 people, add another card of each color to the deck, so there are 7 of each color. I have not play-tested very often with 7, so I assume that the game starts to fall apart a little more, but it should still be playable. The nice thing about cooperatives is that you can have some players control 1 pawn as a team. Our groups tend to make decisions as a group often enough that we rarely notice that there are less pawns than players. If you need to support 7, you can choose if you want to try the 7 pawn rules or the 6 pawn with one team.
These tend to give good results as they keep the game fairly balanced. Having a bigger island suits extra players better as you have a better ratio of Waters Rise! to treasures and more island no-sink time. You'll have to plan a little more as you'll end up farther away from each other. The degradation of the island is more interesting as you can have bigger holes in the center. You can even start changing the shape of the island for extra fun. I will be posting a list of new roles shortly.
Feel free to post any question, comments, or reviews of these rules if you've played with them. :D