Thanks for the comments folks

Keith: deep breath and ..... calm....

So does this wood glue not warp the boards when applied like this?
Simplistically, without framing* there's always the risk of warpage. It's just a matter of eliminating or managing it as part of the overall tile build. In any event, if every tile is equally warped to a small degree then the overall visual effect can be that they are not.
In terms of elimination and management with regard to water features, I favour smaller, square tiles with a shallow pour and with some terrain features as 'stiffeners' to either side. There'll still be a draw with the first pour as it contracts and pulls the sides in but I usually bend the board over a table edge to flex it back as level as I can. It'll be interesting to see how the bay turns out as that is a very large pour.
*So I should frame then, right? Except framing adds weight and depth to the tile. Not so much per tile but I have well in excess of 100 tiles of 30cm sq, ranging from say 3cm depth to 30cm depth.