If you live next to like a bay or a deep ish reviver then I recommend you tame two one for underwater exploration then keep the other ones stamina as low as possible and swim it around the river that way the stamina runs out and they will eat to re gain stamina then they will produce oil because they are naturally behind fed (note force feeding them won’t produce oil they have to eat by themselves)
Good fighter, immune to jellies, supernatural healing boost at the surface, produces oil in its inventory, impervious to squid grabs, looks kinda cute too, only one bad thing about it: it gets squashed in the deep! Only slowly but the damage is there!
This creature is so easy to tame. All you need is:
1. An Ichthyosaurus(these guy won't be attack by any water creature, except the jellyfish)
2. Anykind of meat(I suggest using prime meat and don't use fish meat)
3. Find one of these creature
4. Feed it prime meat(It's a long time between the 1st and the 2nd feed, after the 2nd feed it's fast)
And then you get one of these, just be careful of the megalodons, they will attack right after it is tamed. I hope this is useful for you<3
In real life these were vicious carnivores that would have killed and eaten a human on sight and lived mostly in shallow waters and swam up rivers looking for food they were also twenty metres long, good thing the developers didn't make them more realistic or crossing rivers would be a dream until you have a flyer.