How long do you want the boat to last?
Lowes plywood is not meant to be in the weather. Exterior grade plywood, except for that used as siding on a house, is covered with something, shielded from the weather. It was never intended to be wet. The wood used is not rot resistant, the plies in the plywood are thick and there is less of them. There are hidden holes inside that can lead to problems. It tends to warp. The only thing going for Lowes ply is the cost but that is a false savings because it will shorten the life of the boat.
Sure you can coat it with epoxy but that adds some additional weight and epoxy is not cheap. But if you miss a spot or something happens and you get a hole in the epoxy, water gets into the wood it has no way out. It will stay wet for then on and it will now rot faster than if had nothing on it. If not done perfect it won't last for decades.
This is an extreme case but this is exterior plywood I was going to use for make a form for laminated coamings. I cut it out and left. Next morning this is what I found. Do you want this in your boat?
Baltic Birch and MOST marine grade plywood are made of more thin plies with no voids hidden inside. Marine grades should use higher quality wood meant to be used in wet conditions. The quality of imported plywood is FAR superior to any American made plywood I have seen. Sad to say but this is one play I want imported products over US made.
As for being cheap, Fuselage Frame boats are about the cheapest boat there is out there without being a throw away boat. You should be able to build one for $400 using good materials and have a boat that will last a lifetime with just normal care. That should appeal to any cheapskate.
By the time you buy plywood and epoxy your probably going to be close to $50. Lets just say you can have $40 in both. A good sheet of the marine grade plywood is $100 +/- (I buy Baltic Birch for somewhere around $54 I think). No extra work required to it. So your going to save $60 on a a $400 boat and you will probably shorten the life of the boat.
Yes I am biased, but I think it a no brainier.