I want to buy a go pro but no idea which model to buy ...there are different versions ..

I need to take good underwater videos that all nothing more
So which go pro should I buy ..with underwater scuba diving casing ..
First hand or second hand doesn't matter

If anyone have one and willing to sell then please reply me
Thanks

Hero 4 silver/black will be just fine..  I have a hero 4 silver for sale. Pm me if you are interested.