I'm in the process of builing my pond at the moment. It will be a Koi pond so a little bit deeper than 900mm (7' to be exact) however if you are looking for a sealant that is fish friendly, the best one avilable at the moment is G4, however this is a little on the expensive side.
Have you thought about fibregalssing it or getting a box linner for it (unless of course it is not a square / rectangle).
Also remember that if you are not going to go for a specific pond waterproofer or fiber glass you must make sure that the resin and gel coat are not toxic to fish. In both these cases after the pond coat has cured it must be washed down with soapy water before filling the pond.
Are you going with bottom drain gravity fed or pump fed system, I would highly reccommend a bottom drain?. Also for a koi pond of that size you really want it a little deeper. minimum for Koi is about 4'6".
One other thing, for the surface area you have you will probably need two bottom drains and the bottom benched towards them, unless of course you are looking at aerated bottom drains like spindrifter, in which case you don't need to bench the bottom, only make sure you reduce all 90 degree angle to 45 so you don't get a build up of waste for the bad bacteria to breed in.