First of all, you don't need to carry all 40 gallons. Carry enough for a few days and get more along the way. I don't think you'll have any issue storing it either.

As far as drinking the tank water. It's not going to kill you. I personally prefer not to drink it though since you never know how often the tanks are cleaned out. You also never know what could grow in a tank that may go unused for a period of time. We've had charters where we could have simply used a single tank the whole trip. We end up swapping just to keep the water fresh but not everyone may do that.

Better question might be to ask what kind of storage is on the boat you'll be on.