Humans have lost the estrus signals used by all non-human primates
Chimpanzees, gorillas, and all other non-human primates have signals of sight and/or smell that indicate estrus — the time for successful mating. Marine mammals do not have these estrus signals. They would be difficult to detect in seawater.
Female chimpanzees exhibit remarkable sexual swelling, which would not be seen when standing or floating in water.