I want to build cabinets hanging from deck joists. The deck will have a waterproofing system installed so there shouldn't be much (if any water). What are my options for plywood that can withstand Maryland heat cold (but hopefully little water). I was told (Baltic) Birch, but I assume that would need to be sealed. Maybe for this application, I'd need to paint it and Birch would not be the way to go? Any Suggestions?
Converting my Comments above into an Answer as per a request.
I was told (Baltic) Birch, but I assume that would need to be sealed.
Although it's not sold to be used outdoors, if there'd be no direct weather on them then Baltic birch is actually an option.
But if there's the least chance of rain getting blown under there during a storm, or even splashes off the ground during heavy rainfall, you really do need to go with an exterior-rated plywood for longevity, unless you finish them suitably. If finished well you can actually use birch ply with direct exposure to the weather, as mentioned in some posts online, e.g. here on SawmillCreek.
The thing is, the only reason to use Baltic birch ply is to see it, unless you're just using up offcuts of course :-) It's made to be seen, and that's one of the things we're paying for when we get it, so it's completely wasted if you paint it. You've paid a premium (sometimes a very high premium) for no reason.
So I think you'd be much better off with something else. Proper exterior-rated plywoods like MDO, WBP ply (water and boil proof, means literally what it says) and marine-grade ply may all be cheaper than Baltic birch, which is a cabinet-grade plywood. In addition you probably won't have to finish it for this use case, so cheaper up front, you'd save on buying finish and save the time a thorough finishing job would take1. So multiple reasons to choose something else.
Honourable mention: OSB. This is definitely worth considering if you either like the look (some people absolutely hate it and for them the three letters stand for something else LOL) or this project isn't highly visible so appearance is of secondary importance.
This video from Fix This Build That, 5 Mistakes Buying Plywood - Don't Waste Your Money! was posted just a couple of days before you posted your Question. It sort of skips past exterior ply but as it reflects current pricing it's worth a quick look, and it gives a taster of what's available from common sources, the quality you can expect and the variation there can be in a single category..... as well as from sheet to sheet. So perhaps the number one take-home from it is actually to bring gloves, put 'em on when you get to the stacks and move sheets2 until you find something you want to spend money on!
1 Not to be underestimated. Primer coat, possible touch-up primer coat after inspection for holidays (missed spots), then perhaps three coats of your chosen exterior paint. That's five days start to finish, if you have an hour plus available to devote to this on five consecutive days. Aaaand then you should let the paint dry and partially cure for at least a week more. Realistically this might add two weeks to the schedule.
2 There's a tip on that too :-)