I would just do the same setup you had on your D800, the main difference is focus mode on the Z cameras and that takes a bit of getting used to on people or bird modes where you have the new subject detection modes in focusing. I have played with the people, animal and bord modes on fish shots yet didn't really find a preference so far. Basically the above options either had slow lock on, on fish shots which caused me to loose initial shots when fish went into frame , or couldn't keep track of the fish as it moved which caused me to not be able to track and shoot. So far I find myself going back to single focus where I focus on the fish and take a couple good shots before it moves then refocus if needed. The continuous mode always had its issues for me on fish shots with focus going on to another fish and if I set it to be too sticky then it would take time to unfocused and go back to my fish I'm shooting, setting it to not be sticky on subject causes u to lose focus if another fish or subject goes into frame closer than ur fish and it's tricky !
I'm.not sure about the D800 vs Z50 as I came from the D750 to the z7ii and z8
But believe ur z50 might have a bit better performance at higher iso ? If so then u might benefit from a bit more iso without getting much noise. Which lenses are u shooting with ? The new 50mm for mirorless is great for fish shots but it got double or triple the price of the old 50 mm for non mirorless. The macro 105 for mirorless also got an improvement, I'm also very happy with the 24-70 2.8 but uts a more expensive one. The 24-120F4 which came with the Z7ii was a good lense for the price but focus and sharpness on thr 24-70 2.8 was much better so i switched to the 24-70.
Take many shots and experiment with different settings to get used to the canera , it'll be tricky first few times as button placement is different but you'll get used to it rather quick.