The philosophy my Dad always ingrained in me, regarding tool and materials purchases, what our family referred to as his "Cry Just Once Theory"..... it's gonna hurt, no matter what you end up paying, so buy the best you can possibly afford at the time, that will complete the task adequately and keep to your standards. If you go for cheap, & it doesn't perform its purpose properly, needing maintenance or outright repair....or worse, it fails and you have to replace it, you'll be crying TWICE..... best to "bite the bullet", and only have to cry once.

Most certainly we all have budget constraints and priorities, but try not to let any short-term thing influence whatever course you decide to pursue, at the expense of getting the job done correctly, to your standards, of course.

The deflection of 1/2" Baltic Birch ply is fairly minimal because of its many plies, which may allow you to use less (wider spacing of) aluminum strut work, which could cut some cost. And it's edge consistency, as well as it's near-flawless Birch faces, means easier to finish and maintain.
Just a couple of points for consideration.... good luck !

The most overlooked plywood for such boat projects, as well as many other things, is BALTIC BIRCH (only as 5'x5' sheets originally, but now in 4x8, as well, import from... the Baltic region, of course) and the domestically (Oregon !) produced APPLEPLY (in rather short supply lately & Timber Products won't give a specific reasoning ?!?), similar to BB, in that it is made of millimeter-thick plies (1/2" is 9-ply, 3/4 is 12-ply) almost totally void-free and WATERPROOF GLUE !
1/2" 4'x8' is $105 right now, just called my local yard.
I have been using BB ply for drawers, as well as any weather-exposed plywood needs... like boats !, for 40+ years, no complaints and it's so well manufactured, that it not only doesn't warp or delaminate, but can be machined quite well. Usually just rabbet the corners and glue and pin with an 18ga. gun, but have even "dovetailed" it, using a jig. Also doesn't need edgebanding, as it is "void-free" and kinda pretty as exposed "edge grain".
I would still use some type of sealer for using it in a boat. My go-to for that, is "Daly's Ship 'n' Shore" or Daly's "Seafin", both soak in, rather than form a film that would be susceptible to chipping or peeling, not cheap stuff... but what is these days ?
Find a lumberyard that supplies cabinet makers (NOT your 2x4, 4x4 guys or a big box store)and you'll find Baltic Birch ply, some are available prefinished with UV-cured "lacquer" finish (haven't used that version).

Big fan of the PVC "lumber" (at Lowe's and Home Depot)for boats, but have yet to see any sheetgoods available locally. Spendy, but worth it, imo.
Have used it on 2 of my boats, love its no mold, no rot, no paint (unless you wanna go with something besides white, used it on exterior casing/trim that I painted with acrylic latex, 5+ years on... looks the same as the day I finished) no absorption traits. Even routs, drills and holds screws well.
Between my little "fleet" of boats and all my buddies who want some customized add-ons for THEIR boats, that seems to be where my woodworking skills go these days.
Glad of it.... last big kitchen/pantry/dining room contract, before retirement, had 56 drawers and 89 doors, all told ! Nice paycheck at the end, but damn..... that's a lot of cutting, sanding, finishing and fitting. Don't miss the long hours and deadlines, in the least.
Good luck on your project !

In response to your question, I built my decks completely from Wood because I did not have the expertise to work with aluminum. I used 2X2 for my support structure in my 12 foot Lowe John boat. I will attach photos.

I used 1/2” marine grade plywood that I bought at Menards. The plywood was extremely expensive costing $100 per sheet. I have found a half inch to be plenty strong enough to support my weight, but I only weigh between 160-165 pounds. The front elevated deck makes no noise and does not show any signs of shifting or bowing under my body weight. You can also see that I cut a hole right in the middle of it and installed an access hatch.

I then installed a subfloor between my second and third bench seat and simply laid a piece of half inch plywood directly on top of 1.25” styrofoam. It supports my weight along with that of my grown daughter without any problem. Unless you’re a really big guy, I think you can get by with half inch plywood and save yourself some additional weight as compared to 3/4” plywood.

I sealed all of my wood with a deck paint that I used on my home. I then covered my plywood with EVA camo foam.

I wish you the very best as you build your decks. I’m not very handy with any type of construction so I worked very slowly and just took my time. I know someone else could’ve done a better job but I am extremely satisfied. BeaverTooth

I fished El Cap a lot back in the 90s and early 2000s and did well on bass. Now I only fish the South Bay Area launching at Pepper park, I only catch and release mostly spotted bay bass but also halibut, croaker, shortfin corvina, bonefish, mackerel, and other species. I fish plastics only but have in the past used squid and mackerel mostly catching dogfish, bay rays, mackerel and others. Have never been skunked in the bay, worst day there was 9 fish by myself and best day for 2 of us was 176 mostly spotties. If you’re interested in trying the bay PM me and I’ll tell you what to use and where and how to catch them with good numbers. Or if you want I can take you out on my boat sometime. Larry W.

Hey froggy, I lived in Santee back in 68 thru 71 and now in scenic Alpine for 43 years. I have a 15’smoker craft alum boat that I’ve had for 10 years, and when I got it I made a deck in front and floor boards out of 1/2 CDX plywood and sealed it with 2 coats of exterior sealer and the covered it with indoor outdoor carpeting from Home Depot I’ve changed the carpet once about 2 years ago it was just worn from our shoes not from any moisture. I made some tapered runners for the floor to make the plywood sit level. I left the floors unattached so I could remove them to clean them off. I made a elevated stand for the front deck for my swivel chair, and I can use my front mounted trolling motor from there. Larry W.