Resin alone will not increase the stiffness, and what resin do you plan to use/
I'm not sure why people think they need to increase the plywood thickness up to 3/4" on small aluminum boats, most of the time 1/2" is fine, but it depends on the support structure under it. I'm not pointing at you on this, it seems to be a general theme, if 1/2" works, 5/8" must be better, so 3/4" should be great. Having a small amount of flex is a non issue, and 90% of the time you will never notice it.
The increased weight on a small boat is far more of an issue than a slight amount of flex. There are situations where 3/4" may be needed, but not often.
Use whatever thickness ply was there to start with.