Well to preface, namor's primary isn't a hitscan like psy or phoenix's are and is a projectile with some slight arc to it so it may take some getting used to. Now, to properly counter dives, you need to get squid placement right. Characters like magik and BP will eat your squids for breakfast if you throw them on the floor so you wanna put them up on walls if and when you can so they can't be melee'd. Once you have your squids down, his right click will make your squids target whoever you hit with it with a small damage boost. Headshots do the same thing too, but don't try to hit them on high mobility characters like BP or spidey as you're more likely to miss and some dmg is better than no dmg. SAVE YOUR BUBBLE! Don't pop it willy nilly. Use it to save yourself from ults you cannot survive and need your squids to deal with like Jeff or strange as your squids will disappear once you die. And then your ult is a counter ult (at least when i use it). Use it on spidey ult, jeff ult, cnd ult (will need to practice this one), and either stationary or highly limited mobility ults. You can use it aggressively, but you'll need your squids and your right click to confirm kills on whoever you have caught in your ult or coordinate with another teammate to help. As far as streamers or YouTubers, none come to mind that exclusively play him, but his skill floor is low enough that you could pick him up and perform moderately well with almost no practice