Most likely, you NEED either flippers or an aquatic mount. They get pretty interested on you and your creatures at first, but once you touch them, they switch to the approach and flee behavior usually seem in aggreasive creatures. If you are fast enough, however, they won't reach you and you can now kite it to your trap, just close it as fast as you can.
Another option is finding / waiting for a beached specimen, so you can just place some gates around it.
Best for killing scorpions on bottom of ocean floor since they don't take torpor. Scorpions down there will knock any other animals out. Once you kill it, hit it with a pickaxe to gather black pearls. Very useful for the next update with tek gear.
Since there's no way to post it on the actual creatures wall I think I should warn everyone whilst taming these beautiful ichthyosaurus watch out and I mean watch out for any Cnidaria jellyfish they will shock you to oblivion I was caught taming when 4 of these guys jumped me and I absolutely freaked out
So yeah Cnidaria are easy to kill but keep your distance!