Thanks to all.
Underwater: Is it hard to do? That's rather relative. Given an ability to understand and control depth of field and to consistently produce properly exposed and in-focus images, it's rather easy to do. If you can't deliver good quality single-shot images, stacking is just going to accentuate any flaws and probably result in a mess. I'm not sure what your skill level is and don't mean to insult you, but I don't want you to be deceived.
You need software to do the stacking. I use Helicon Focus. There might be other packages on the market, but I've not researched them so I don't know for sure. Helicon Focus gives a 30-day trial so you can give it a try to see what results you get.
It's fun to do and there's no harm in trying. After all, it's only data. If you don't like the results, hit delete.
Gary