Doc - are you going to paint the new transom ??
if so, just dilute some of the oil-based paint 50/50
with mineral spirits and that will be your sealer. (skip the linseed oil).
48 hours later, a coat of the same oil paint reduced 25%
then 48 hours later, the same with 10% reducer followed
by a coat or two full strength the next day or so.
We have done plywood signs that way for the last 75 years
and they hold up well...... the varnish/linseed oil/spirits mix
works just as well, IMO. Lead based paint works the best, but,
we know where that went !!!!!
as for the glue,
TiteBond III is touted to be "water-proof".
a good slathering on both sides of the plywood and a full
wet coat around the edges to help prevent water intrusion.
don't cut the panel to fit first - cut over size, glue it all together
with the two held in place with a couple of 1" wood screws.
put several layers of newspaper on the floor and put a ton of weight
on it to sit over night........
if you use clamps all the way around the edges, you will force all
the glue into the center of the panels and may not dry/cure/bond sufficiently.
with the weight distributed equally on the panels - the excess glue
will be pushed out of the two panels......
then - cut to shape - seal and paint.
Depending on how you store your boat, how you use your boat
and what part of the country you live in determins the degree of preservation.
unpainted bare wood in Alaska will last 10 years or more with no problem.
unpainted bare wood in Georgia will last one year or less.
Your Boat = Your Call
.