I was hoping someone else would answer this post as some builders can take a fairly hard line about the quality of materials and feel that only marine rated plywood should be used. Personally I am a little more liberal. Even so, I would never argue against the concept that Marine ply is almost always the best. BUT, in my opinion, the individual builder has some latitude depending on what is being built and how it will be used and the expectation of its longevity. The questions I would ask myself are -
1. Do I intend to use the boat a lot OR just something to mess with from time to time?
2. Will it be on the water 24/7 or stored outdoors OR will it be stored under roof?
3. Am I building a heirloom I want future generations to enjoy OR just is this something to knock around with for several years of fun.
To me these can help guide the decision of the quality of the materials you purchase. If you answer yes to the first part of any
of the questions I would suggest sticking with marine grade plywood and quality materials through out the build. Also, of the 3 questions, I feel #2 is the most important. A boat that sits on the water a majority of its life OR is stored outdoors is subject to far more hazards than one stored under roof. Even the best trap or cover can develop a leak and water sitting in a wooden boat for months during a storage period can cause serious if not fatal damage. Quality materials may not prevent this but usually will substantially reduce the damage.
On the other hand if you fall in the the category answering yes to the second part of the questions you "may" be able to make a few substitutions and still build a sound craft. The hull of my Cabin Skiff was built mainly with a nice grade of exterior plywood (12 years ago you could find nice exterior plywood - today, not so much!). It has seen a lot of use in the past 12 years and in my estimation is still as sound as the day it rolled out of the shop but it does spend the majority of its life on the trailer and in a shed. So I feel if the situation is right you can hedge a little.
As to luan - First I would want to be 100% sure it was constructed with an exterior glue. Most that I have seen are not and unless it was exterior I personally wouldn't use it. All it would take was a small unnoticed scratch in the bottom to allow a little water intrusion. And it often doesn't take much moisture for interior grade plywood swell, delaminate and cause all sorts of problems. When encapsulating the epoxy does soak into the wood somewhat but no way deep enough to affect and protect the glue lines of the plywood layers. Also any exposed edge of any plywood is a prime place for water to enter no matter what type of ply is used.
Plus laun is not designed for lateral strength. Ofter it has a thick core and very thin outer layers. Also usually there is very little attempt to position the core to add rigidity. A few years back I went to the local lumber yard to buy a sheet of 1/4" luan for a home project. I picked the sheet up by grabbing it in the middle with a hand on each side - it promptly snapped completely in half. Obviously the inner core had a splice all the way across the sheet and the very thin outer layers would not even support its own weight. This may have been extreme but you must remember what laun is designed to do - provide a smooth surface OVER an already solid surface. It doesn't need to be strong.
Another alternative might be MDO (medium density overlay). I used it extensively on my True Grit for the topsides and cabin and I have read some builders have built entire boats with it. I have a posting about its use - good and bad - at this link http://www.egyptian.net/~raymacke/TG/MDO.html
. Might be worth taking a look.
And again, all of this is just my opinion and I totally recognize there maybe differing views. Actually that is what makes forums like this such a great resource.