Any time you have warm moist air meet a cool surface you'll get condensation. In the Northeast, the water
and air temperature can be in the 50's even in July and, if the boat is sealed up with warm breathing bodies inside, moisture will condense on the inside of the hull and overhead. The best way to avoid it is to have a lot of ventilation. A heater (vented to the outside) also helps. I'd think condensation would be less of a problem in LA, because both outside air and water
would be warmer.
The real problem with condensation is that it promotes the growth of mildew. For this reason, a FG liner is good because it's easy to wipe off and clean. However, as I already pointed out, retrofitting a glass liner would not be an easy project
. Once you get mold in carpet or foam, it would be very difficult to get rid off. Wood is a very traditional liner for yacht interiors, it's relatively easy to keep clean, and relatively easy to install. Cedar is good because it's lightweight and resistant to rot
. Other woods to consider would be redwood, mahogany, cherry, etc.
Small boats being what they are, it's probably unrealistic to expect that things will never get wet (especially if you actually sail, as opposed to stay at the dock). If your mom wants to keep some things absolutely dry, I suggest a generous supply of Tupperware containers, ziplock bags, or dry bags like one's used by kayakers.