I went to basilo blubber gathering to Crystal Isles tropical area with a desmo and a great shotgun. There are not too deep waters.
I really found out the laziest and easiest method, and you need to do it at night. I don't like to be in water ingame, and don't have any aquatic creature to battle with them face to face...
So I just turn on the desmo's night vision mode, looking after the basilos, and shot them from the air. When I start shooting, all the mantas flee away, so when the basilo is dead, I can just swim to it for it's loot.
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.