If trying to tame one make like a wyvern trap on the bottom on the ocean and hive someone else shut it. I use a sarco to lead it in the trap. On the center you can make the trap right next to the air bubble to escaped in the Airr away from any attack range and have a keypad to shut it. This will allow you to use any tranq attack
1. Don't shoot their head, shoot their butt or body. (This is true, dont trust gamepedia)
2. It wont left deep sea, so u can easily stay out of deep sea border and KO them.
3. Make sure to kill everything else around it before start to K.O it.
Build a 3x3 trap on a plesio patform standing on pillars as far back and high as you can go.
Dunkleo and sharks won't be able to attack plesio from inside. Trank risk free. Swim away when it fell asleep. Similar to quetz, just no need to have back wall removed/placed to catch it, the will swim thru the wall if you move. Once inside , just stop moving and it's trapped.
It will take around 10ish minutes to knock a high lvl out. You must bring 2.5x the arrows it says will knock it out and extra crossbows. Extra scuba tanks are also recommended since you'll be in the water for quite awhile and the first will most likely break. Once you start shooting it it will try to flee but is easy to keep up with. This is why you must clear out the area around it and keep it in the area or you might be lead to predators. It will take you on a journey if you don't corral it in an area. Occasionally it will turn around and take a bite out of you and your tame then will go back to fleeing so be vigilant. After doing all this, just keep firing away until it finally goes down.
Make a stone or metal box underwater, two walls high and three or more walls wide. They flee if you use any narcotic arrows or tranq darts so avoid those when kiting one into the box. Once inside, they cannot escape through a gap one wall thick or high.
Easiest way to tame a dunkleosteus is to build 2x3x3 with ceilings. Attack it with reg crossbow bolts til you get it into the enclosure. Make sure you leave one square open in the back to escape through. Once in the enclosure the Dunk will attack the stone foundations and wall and while doing the you can close off the other end leaving one space open. It cannot escape and the body glitches through the walls for easy tranq shots. Easiest tame ever