Ya, you could go the swiss cheese route, but chedder is bedder....
Boring some holes is one way to do it. What I like to do is to cut squares or rectangles. I'll use a ruler to measure off 1/4 - 3/8" all the way around the former/bulkhead, like you were going to make the part smaller. Then I put 2 upright portions, or 3, to keep the up/down strength and cut out the rest on my jigsaw. ( imagine a ladder...you have the sides and the rungs, all the space between was cut out) This still leaves the part with strength, but a lot lighter. The whole framework interlocks together for the remainder of the strength. It may seem flimsy at first, but once the deck and bottom sheeting goes on, it stiffens right up.
If you use lite ply, go with 1/8" for all the frame members and 1/16 ply sheeting. If you use regular ply, you can go down to 3/32".
I built a few hulls with a balsa/ply combination. I used 1/16 balsa backed with an identical piece of 1/16 ply, also 3/32 balsa with 1/32 ply. They worked very well, but more work is involved