We're using pebble tile from Stratastone for the floor. My design idea is to have white unglazed pebbles
with a 'meandering river' of green/blue glazed pebbles through the center of the floor.

Trying to figure out the best way to layout a non-geometric type of pattern like this? And then, how do I transfer that to the underlayment so it can be seen by the tile installer as there will be a mesh heat mat on top of the underlayment. Can I use a black sharpee marker or is there something else? Thanks

Also, does one need to seal the unglazed pebbles as with regular tile?

Instead of marking it on the floor, mark out the dimensions of the floor onto cardboard (old boxes), create the stream pattern on that with the whole and partial sheets, and have the tiler move the tile sheets to the bathroom to lay them.

Do it close to the bathroom, of course.

You can use bits of hot glue to hold stray pebbles into place on the sheets.