Hi, I'm a member here, but my day job is in the music industry.
I work for a company that makes these... Among many other products. Not trying to advertise, but to help out.
Amazon.com: Planet Waves Humidipak Automatic Humidity Control System (For Guitar): Musical Instruments
Normally, you'll have to worry about humid conditions. It's best to keep your instrument in a case when you're not playing it, and try to control the environment in the case.
Humidipak will regulate in either direction. However, in dry conditions also use another moisture adding device like a Guitar Humidifier so the pak doesn't dry out.
If its really wet conditions, (above 70% day after day) use a "dried out" humidipak in your case which will absorb moisture.
Again, keeping the guitar in a case when not in use will really help. Also, go to your local shop and learn how to make a truss rod adjustment because the neck will move. It's perfectly normal, I adjust many of my instruments every year to get the action right. It's routine and if you can do basic boat maintenance you can do basic guitar maintenance. Wood moves with trends in humidity...
Finally, consider coated strings to minimize corrosion. This is vital on a boat. My company makes the EXP line of strings but there are others out there as well like Elixir.
If you have any questions, don't hesitate to ask. I know Cruisingdad has a guitar on board, perhaps he wants to share his experience.
Good luck, keep playing...!!!
D'Addario and Co.
I use D'Add EXP and they are awesome! It is the only string I will keep on my guitar permanently (though I use your cheaper D'adds when I break one). I have also noticed that they last longer from corrosion. For example, I broke an E a while back and the spares were all the non-coated D'adds. I played with that for a while but noticed within a few weeks the corrosion on the string where I pick. That string is now off and replaced with another EXP.
I am a bit miffed about the comments between the PNW and Florida. My experience is that the PNW is very dry - though it rains all the time. Weird, and doesn't make sense, but that was my experience. In fact, it was not unusual for us to put my sons Cello in the bathroom while he was showering to add some moisture to it before a concert. I was more concerned about keeping my guitar in PNW than in Florida! I have always been under the impression that moisture (not too much) is good for wood instruments and helps their voice. In fact, I have had people tell me that one of the worst places for wood instruments are places like Arizona where it is dry and hot.
The exact opposite is true in Florida where we have very high humidity... maybe too much and too much heat at times. SO, if I am not going to be playing the guitar for a long time, I loosen all the strings (just like if you were gong to take it on an airplane). I have had the neck on my Eastman warp about the tenth fret, but I think that was more an issue with the guitar than the environment.
I currentlty play a Taylor 814ce. It is one of the finest guitars I have played, with the exception of the 914ce which I like better and the Martin X(I forget which one, but comparable to the 914... not the GPCPA1). I am a huge Taylor fan - especially in this environment. The neck is fast and REPLACEABLE. The cases are very solid and tight. THis guitar has thousands of miles on it (especially adding in the airplane trips) and not a single issue. We live aboard and the guitar sits nicely beside my bed in its case. I do think that is the key: keeping it in a GOOD case. I do not use any moisture control... have not had to. We do have air conditioning on our boat, and when it is "too hot for the guitar" it is likely too hot for us. We will either be in a marina or running the generator. THat could be a long term issue though - as I generally play in the cockpit (lot of moisture) but keep it down below (dry when the AC is running). No problems yet though, and it has been here for almost a year.
Go price out the difference between replacing the neck on a Taylor versus a GIbson or Martin. Big difference. In fact, on some guitars, replacing the neck ends up being more than the guitar is worth. I found that out on my Takamine. THe Luthier I used also was not excited about replacing the neck on my Eastman, which I value around $500-$600.
I would not hesitate (and have not) taking my guitar on the boat. I love it and have had no problems with this guitar. I think it is a lot better for the guitar to sit on a boat and get used than it is for it to sit in storage and collect dust. Just keep an eye on your neck by pressing down on the 10th plus fret and making sure the height (action) stays consistent down the neck. A neck problem often shows itself early with a small decrease in the space or a buzzing on the E's.
Anyways, I am not an expert in this field. I only enjoy playing and lean on the advice of others for their opinions (luthiers). The only thing I might suggest is to use a really good (hard and tight) case and not a gig bag and keep an eye on your neck. OUtisde of that, jam away!
PS Love the Gison J150. That thing has a voice!!