Minecraft question!

So here’s the backstory: When I was boating looking for a good cave, I saw a zombie with full set of diamond amour and a zombie with a sword. It followed me and fell in the ocean. I never saw a zombie become the drowned so I decided to wait a little and have a look. When they finally became the drowned the zombies dropped all their stuff!

So obviously I jumped out of my boat and picked up the amour and sword. I was so stunned that I couldn’t get back in my boat so the drowned started to kill me. I got back in and went back to my village. Checked the amour and it had full durability and the chestplate and leggings had blast protection and unbreaking. Same with the sword but none of the protection and unbreaking.

The Question: Was it rare for the zombies to drop all their stuff when they became the drowned or was it guaranteed? And how rare was it to find a zombie with a full set of armor?

Thanks for reading and if you wish give me some tips to progress! What to do with the armor?