These are encountered on several rally stages and will eventually go through 200 water-splashes or river crossings, the co-driver also calls these out.
To speed the amount up I found Training C3 in career to be very helpful. It is a small dirt oval with 3 water splashes per lap, which is about 18-19sec per lap. I'd recommend just staying on that training course until the achievement pops as it is way faster than finding them on stages.