/** Apply ocean/canal speed fraction to a velocity */
    uint ApplyWaterClassSpeedFrac(uint raw_speed, WaterDepth depth) const
    {
        bool is_ocean = depth >= WATER_DEPTH_DEEP;
        WaterDepth depth_diff = WATER_DEPTH_MAX - depth;
        byte speed_frac = min(256, is_ocean ? this->ocean_speed_frac + depth_diff : this->canal_speed_frac + depth_diff);
        /* speed_frac == 0 means no reduction while 0xFF means reduction to 1/256. */
        return raw_speed * (256 - speed_frac) / 256;
    }