I have two fixed buoys at my swim spot in the lake that I use when I do intervals in open water. I know the distance between the two buoys so I have a better estimation of the distance that I swim each interval and I can hold onto them during the rest periods making it a bit more relaxed. Maybe you can find some buoys or other markers as well?
But in general, in my opinion you will always compromise a bit on the quality of the intervals when you do swims with speed work in the open water. It’s just something you have to accept as the cost of being able to have your swims in open water 🙂