Infamy is similar to GTA's Wanted levels--as you kill humans, you gain notoriety, and shark hunters will pursue you. When you destroy enough of their boats, a named bounty hunter will show up and you can kill them for an Evolution.
This was taking me forever as I was targeting the shark hunters and not their ships. While eating the hunters is good for topping off your health, it takes ages to fill your infamy meter, and your primary focus should be to sink the shark hunters' vessels. Use Tailwhip

and Lunge

to hit their boats, powering up with your Active Ability

for additional damage when needed (you unlock Active Abilities later in the game when you receive Body evolutions). Use Evade

to dodge if divers or hunters lock on to you. Jump

out of the water and onto boats if you lose health--you can grab hunters to snack on as you tailwhip the deck of their ships. Back off and eat some nearby fish if you take a beating.
The final 2 bounty hunters have electric shields, but you can just go under their boats and tailwhip the shield emitter with

. It should break after a few shots.
I have 3 suggestions for gear that will help you tear through all 10 infamy levels with ease:
1. Shadow Teeth are OP and practically break the game. They give you additional health every time you land a bite, and you can bite things much faster than they can dish out damage. This was the first thing I upgraded to Tier 5 after my Sonar and it made the rest of the game a pushover. You get the Shadow Teeth as a reward for finding all the locations in the second area, Dead Horse Lake.
2. Use the Bone Armor for the rest of your body--Head, Body, Fins and Tail. This gear is made to decimate boats and will help you grind this out quickly. You get these from killing Apex predators.
3. Equip the Hearty Organ, which is unlocked after beating the second bounty hunter. It gives you a great health boost and helps prevent you from being knocked off boats.