Gentlemen,
I built both my water tanks as well as the two diesel tanks from cheap
(rough finish - best) half inch construction plywood and simply used
the same ordinary epoxy resin and glass cloth (one layer of 11 oz
cloth to the inside) I used to clad the hull and decks. I did apply
epoxy filler fillets in the corners before glassing. I also glass
taped the outside edges for extra strength but only resined the
outside.

We launched the boat in 1993 and the tanks have not leaked a drop of
diesel and we have done quite a bit of pounding about the seas since.
I did apply about three coats of resin and sanded lightly between
coats.

One of our 27 gallon (imperial gals) water tanks leaked a year or so
ago from one of the top joins. It has only a single side to side
baffle and the sloshing of the water in the half empty tank must have
opened up the joint. Repaired now but I should have glassed the inside
and outside joins of the top more heavily - with chopped strand mat.

Peter
N.Z. yacht Herodotus