I am at the fish monument and there doesn’t seem to be any puzzle nearby that suggests it will be used to activate the monument. Am I missing something obvious?
The puzzle for this is a good bit away from the monument itself. Speak with the man at the scuba diving hut. He will be standing on the boat. Take the tour and go diving. You will find a statue with 3 fish on it under the water. Around the edge will be 3 pillars. One requires you to pull an anchor and the other is broken and needs repair.
Use the clown fish to activate the pillars and once all 3 are done, the fish monument back on the island will activate. You will need to purchase the sprint ability before you can get across to it after the bridge breaks.