DMG page 112
In the DMG on page 112 it lists rules for falling into water. If the water is at least ten feet deep, the first 20 feet of a fall do no damage. The next 20 do 1d3 subdual per 10 feet. Then each additional 10 feet does 1d6 real damage.
For 50 feet it would therefore be 0 (for the first 20 feet) plus 2d3 subdual(for the second 20 feet) plus 1d6 real(for the last 10 feet).
I am not sure whether it is 1d6 every full 10 feet fallen or whether 1d6 for every 10 feet increment means you round up (falling 15 feet is in the second 10 foot increment so 2d6 damage).
Either way your 35 foot trap will do either 1d3 or 2d3 subdual damage (0 for the first 20 then 1d3 for next two 10 foot increments).
(house rules discussions now)
The last time this situation came up in my campaign, I didn't have the rules in front of me at the time and my ruling was that landing in water was a flat half damage. It was easy to implement, remember, and kept things going. It also meant there was a significant difference between falling from a crows nest onto the deck or falling into the sea.
In general, I don't like how the flat progression of falling damage means most characters and villains can shrug off multistory falls without a second glance. In my campaign I changed it to accelerating damage so that each 10 ft increment does 1d6 more than the last 10 feet. For example, 1d6 for 10 feet, 1d6 plus 2d6 for 20 feet, 1d6 plus 2d6 plus 3d6 for a 30 foot fall, etc. So falls are now a serious matter IMC which can have significant tactical importance.