The Mosasaurus is the fastest wild water creature, as well as the most powerful. Even with Scuba Flippers and 140% speed, you won't outrun it. There are a lot of ways to get one trapped, but if this is not a option or you are feeling adventurous, you need, aside from the previous attributes and items, Focal Chilli to increase speed and a good set of Flak Gauntlets/Leggings to provide defense if needed. With those, however, you can even chase it when time comes and it tries to flee!
Bring multiple Scuba Tanks since you'll definitely need.
Just Tame A Tusoteuthis And Drag It Up To The Surface Cause If Not Je Will Continue To Attack But Once You Get Him Up Above There Spawn Hight He Will No Longer Attack But If You Let Him Go He Will Swim Down To The Bottom Of The Sea And Yes This Method Takes Longer Than A Harpoon Gun With Tranqa It Is Safer And Is Not As Exspinsive. I Hope This Helped You Out :)
It’s actually not that bad to tame this thing. The only problem is the ocean since everything wants to kill you. But if you clear everything, you don’t have to worry about it. Bring lots of tranq arrows and narcotics these guys have pretty high torpor and it goes down kinda fast but not that fast. If you have a Basilo that can tank, use them. If you have a Baryonyx that can tank, use it. Just watch out for jellyfish