Hi all. Im a new DM and relatively new to the game. Ive just started a new campaign using Ghosts of Saltmarsh. The plan was to pretty much follow it to the letter but with maybe an increased number of bad guys because i have 9 playing. 

As we are beginning the game its pretty clear that one of the players has basically read through the Ghosts of Saltmarsh and is following the script rather than allowing players to work things out. This is forcing me to change things on the fly to an extent but also taking away from the adventure. 

How would you more experienced DMs handle this?

Change up things. 

Swap out monsters with different ones ( same CR, and still make sense from the Narrative perspective; no swapping out Kobolds with Dragons ).

Re-write NPCs so that they still have the same role in the story - but change their race/age/appearance/personality. Give then NPCs some extra goals and requests not in the original adventure.

Add in custom side quests that you make up completely on your own.

Re-arrange maps of locations.

Just some ideas off the top of my head. Make it clear to the Player(s) that knowing what was published isn't going to be a good indication of what you'll be throwing at them.

I would take a plot line encounter and move the location and then homebrew it with approximately the same treasurer.  Then you need to homebrew an encounter in the location this treasure was supposed to exist.

If this player "knows" the party should go to that place and leads them there and then everything is all different, he will have to doubt what he thinks he knows and the other players will think, "Why did we let him tell us had to come here?"  Then you  can get them back on track with the needed clues or treasure they need by guiding them to the homebrew so they can obtain the information and treasure they need for the story.

You might have to do this a couple times, I haven't read that module so I don't know how many "parts" there are to the adventure.  Good luck.

Make a few changes to monsters and locations.

Also, if you have just started, you can change the NPCs and their motivations. 

Published modules are ideas and guidelines,  take the ideas and make your own world around that.  It only takes a few interactions for things to begin wildly diverging from what was written -- and that's the natural state of running a game (at least to me).  You create the world, then let your players explore it, and you provide the tension or other force to get them started on their journey.

Turn every known 'Solution' into a trap, and punish heavily.  :D

Move encounters to different locations, have different people know different information, put traps and false information in their place.

And most importantly, talk to the player who has read the campaign and ask that they let the party figure it out and play their character as if they don't already know the answers.

You may have to let player go if they are too much of a problem. You have a lot of PCs as it is. Having a Co-DM is another option (especially with large groups). They can play some of the NPCs.

I would second having a chat with the player in question. If they have read through the module, they should try very hard to not let that knowledge influence the choices they make. In addition, you should mention that you may change things in the module in terms of encounters/treasure/monsters and other items such that trying to use knowledge from reading ahead might be of either no use or actively harmful to their character or the party.

Just changing things without discussing it will leave you with a problem player saying "That isn't how it is supposed to be" .. you head this issue off by chatting to the player in advance and letting them know that their knowledge may be useless and even if there are times when it might be useful it is best to pretend that you don't know what might happen next.

While I think having a discussion with the Player is the least antagonistic - and thus probably the better - way to handle this, does anyone here actually think the Player is confused, here? That they really didn't realize that reading the upcoming adventure cover-to-cover ( if that is what they did ), was something they weren't supposed to do? That they didn't expect to extract some sort of game advantage by reading ahead?

I'd be totally fine with the Player saying "That isn't how it's supposed to be". I'd come back with "Yeah? How do you know that, exactly?".

I would totally approach the player and address the metagaming.  It would be tough for most players who are familiar with the adventure to be able to Golden Rule it like a champ.  It would take a special player to do that.  I would talk to the player, explain the situation his knowledge has put the game in and see if he will be able to continue playing or sit this one out.

To be perfectly frank, the last time someone did that in a game I ran, I asked them politely to leave. There is no good reason for a player to read a module before playing in a group that intends on using that module. If they want to look at stuff that's already happened, or check it out after the module is wrapped up, cool. 

I am going to echo the majority of the answers already given.

I have had the same situation myself and I handled in 2 steps.

The first step was to swtich it up change the monsters and location and encounters but with the same outcome.

The player in question could not help but meta game and mentioned quite vocally that I was not following the adventure.

at that point step 2 swung into play.

Why whatever do you mean? I asked to prove that the player had indeed read and was meta gaming. At which point I paused the session and took them to one side and explained the situation and how by them meta gaming it was ruining and railroading the session for the other players.

I think that some players in truth do not realise that they are meta gaming and think they are doing what feels natural, but then there are other players who purely play the game to win and meta and they either need to be encouraged to play the party line or gently asked to leave.

It is a tough place without knowing your group and how close your friendship bonds are.

good luck with your adventure and remember it is a game to be enjoyed and have fun with others