Morning 002
Hour 4: X.E.23 -> X.D.23 (plastic cup)
Hour 5: X.D.23 -> X.D.22 (small, freshly-dead fish - looks like a species of anchovy)

Senku's short jaunt in the water is uneventful, and he's able to tread back to shore relatively quickly.

Hour 6: X.D.22 -> X.D.19 (medium-sized shark tooth)

There's another large river here, but so is the reef. Back into the ocean you go, Senku.

Hour 7: X.D.19 -> X.C.19 (jellyfish - still alive and small, easily avoidable)
Hour 8: X.C.19 -> X.C.18 (small chunk of coral)

Enemy River #2: Defeated.

Day 002
Hour 1: X.C.18 -> IX.FF.15

Senku first sees - then eventually runs into while he follows the coastline - a large large S.O.S. written in the sand. It looks a little faded, like it's been here for a day or two.

There's also another river to the immediate northwest, also large and fast-flowing.

Pausing here in case Senku wants to follow up on the weird writing on the beach; he has 7 hours remaining until Night 002.