Hi mainestreet,
I have to say I’m somewhat envious. I’ve toyed with the thought of living on a boat from time to time … and it’s something that is on my bucket list … maybe in retirement (if that ever happens!) 
I would tend towards a more reliable supplier… particularly for an “application critical” project like a boat mattress. You can see some of my thoughts about why in post #2 here and post #2 here and a search on FBM will bring up more comments (if you need them). There are some more reliable polyfoam sources in post #4 here if a local foam supplier that sells polyfoam at a reasonable cost isn’t available to you. Some of these may only ship thinner layers that can be shipped UPS so check for different prices and shipping policies. Some of these have some very good polyfoam information on their sites.
There are some manufacturers that specialize in making mattresses for boats and the variance in shapes and thicknesses that may be necessary. In some cases (as I’m sure you know) they can lead to some very “non standard” mattress sizes and shapes. I’ll include a list of some of them here so you can call them and “pick their brains” about what may be more suitable for marine application because of the humid environment because this is a fairly specialized application. All of these are not local or even close to you but you may be interested in looking at the different websites and ideas about marine mattresses. They include …
https://www.foxmattress.com/ Holly Hill, FL (A recommended member of this site)
http://www.rockymountainmattress.com/boat-mattresses Online (A recommended member of this site)
http://www.sandiegomattressmakers.com/ (A recommended member of this site)
http://www.rcb.ca/main.asp surrey, BC Canada
http://www.zenomattress.com/ Ft Lauderdale, FL
http://shop.crewlinens.com/bluewavemattress Ft Lauderdale, FL
http://www.hmcwest.com/ or http://www.boatbeds.com Ft Lauderdale, FL and Costa Mesa, CA
http://www.texasmattress.com/ or http://www.antiquemattress.com/index.htm or http://www.marinemattress.com/ Houston, San Leon TX
http://www.boatmattress.com/ or http://www.portlandmattressmakers.com/ Portland, ME
http://www.comfortcustombedding.com/product.html Plantation, FL
http://schraderbeds.com/ West Palm Beach, FL
http://www.seattlemattress.net/ Seattle, WA
http://www.artisansmattress.com/ Saginaw, MI (authorized custom re-sizer for Tempurpedic)
There are many more but this will give you a representative sample and give you a few people to talk with to gather ideas and on the advantages and disadvantages of different materials.
In terms of “regular” outlets … there are not so many great choices in the Brunswick, GA (or Jacksonville, FL) area. Some of the better choices in Jacksonville are in post #7 here but these would be “regular size” mattresses. If you need more options or some choices that are more local … I’ll take a look a little later tonight and see if there is anything right in the Brunswick area. Given the specialized nature of the mattress you need and the environment it will be exposed to though … I would probably suggest taking the trip to Holly Hill and visiting Fox mattress since they are the closest manufacturer I know of that specializes in marine mattresses and I also know them well and think very highly of them.
Phoenix