OK. So you want Units to prefer water, when they have to choose.The only one is depthmod which says water does not slow me.
You want depthmod results that give a final speed greater than a pathing result on land to be weighted higher.
Problem there... IIRC, the pathfinder doesn't actually look at Unit speed, it merely looks at slope. Could be wrong there, but I don't recall seeing anything about the relative Unit speed per se, it just tries to solve for the fewest changes in slope.
I guess water could be treated as a bonus to the slope values or something. That might be a good solution.