I think he's just joking by pointing out someone being gagged and someone drowning as something actually happening rather than just in a voice acting way.

I've never tried doing any recording for voice acting, but I would think at least the gagging scenario would be something you should set up by covering the mouth or whatever to make it more naturally sound how you want it to sound. The drowning one, I don't really know. It would be worth trying while holding a drink to the mouth or something, but I'd say it depends on what you are doing. Is the person speaking while sticking his/her head out of the water before drowning? It might be easier to just have the person say the lines normally (though acting that he/she is continually sticking his/her head out of the water and going back down). Act out saying the lines, cutting yourself off for when your head would go back into the water, etc. Edit anything necessary in post processing. That might be easier/better than trying to find some way of simulating it more naturally with a drink.

Actually, the gagging scenario might be better to do in post processing as well if its important that the line is heard clearly. Maybe do it through clenched teeth or something, which could maybe give the sound of using a little more effort to speak, but you can hear what's being said clearly. Then edit it afterwards to muffle it a little or something

Keep in mind, I know nothing about voice acting or anything, so you can probably disregard all of this.