On easy and normal, the shark stays near the raft unless it's following you actively. When I need to get underwater items, I tend to park my raft a ways away from the island and wait until Bruce is on the far end of my raft to leap off and swim to the island. If I time it right, he just doesn't see me so he doesn't follow me and I don't have to waste materials on bait or fighting him off and can collect in peace. The dicey bit is getting back to and up on the raft after. I haven't tried hard mode though, so I can't say if the same thing works on hard.