Very easy to tame if you know what your doing. First tame an Ichthyosaur. Then grab a crossbow and some tranq arrows and find one and knock it out. It is not be able to attack if you are on the Ichthy. As you are taming it, be sure to remain on the Ichthy unless feeding narcs, so other creature don’t hurt you.
For Fjordur: actually very common spawns, but only at the bottom of sea trenches. When prepping to tame, be sure to clear out any sea scorpions or Megs nearby, or you'll be making drunk fish food.
X-Dunkys took me a lot more tranq arrows than Dododex would suggest, especially considering my 745% crossbow - that still took around 30 shots. Body shots are ideal, since the head is heavily armored.
Be careful starve taming! They don't wake up fast, but they eat very slowly, and mine wouldn't eat the last kibble piece when I dropped in all the ones dododexs calculator said. Let it starve an extra hundred or so food to avoid that happening to you!