Given the same quality of construction, in many situations marine plywood
will last longer and require less maintenance, but you pay more for it. So
there's a cost vs longevity tradeoff. For example if you are buildign a
large boat which will be in the water all the time, or a small boat which
is left outside on a trailer, and you want ot pass the boat on to your
children's children, then marine plywood would be cost effective.
If you are building a small boat like a canoe or kayak which is only
in the water a few days a year and is stored indoors, say hanging from a
garage ceiling, the rest of the time, then marine plywood would not likely
be cost effective. As a rule of thumb a marine plywood boat will last 25
years while a non-marine plywood boat will last 10-15. On larger boats
where weight isn't so improtant, at least 3 layers of "fibreglass" on the
hull turns a plywood boat into a "fibreglass" boat and can increase its
lifespan, with ocassional resurfacing, indefinitely.

Exterior grade meranti (or lauan) weighs the same as marine grade okume
(half a pound per square foot for the 1/5 inch plywood commonly used on
small boats). Here in Ottawa the meranti is $14 a sheet while the okumne
is $50. Meratni is stable unlike exterior grade douglas fir plywood which
swells and shrinks with changes in heat and humidity and developes surface
cracks (called "checks") on the surface which spoils the appearance,
especially if stored outside in the sun. Fir plywood pretty well has to be
covered in moderately thick fibreglass to stop the checking.

I'd avoid the virola underlayment sold at Home Deopt in this area. It is
the grey sapwood, not the white heart wood, and does not last. I tried it
on a small boat (Delta on my website) and have had to do more maintenance
than on the two lauan boats I have. Meranti has replaced lauan in this
area. The quality is much better, no edge voids and both sides smooth.
If selecting meranti try to get sheets without any surface splits filled
with wood filler.
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