Story from the 1950s/60s/70s featuring a dangerous water creature like a yellow hammerhead

"Grandpa", by James H. Schmitz. Originally published in Astounding Science Fiction, February 1955. As mentioned in my comment above, an online copy is available at https://web.archive.org/web/20150826151128/http://www.baenebooks.com:80/10.1125/Baen/0671319841/0671319841___2.htm .

It's been anthologised in numerous places, and I've seen it so many times, which is how I found it. I have in front of me a copy of Spectrum V with it in. But equally well-known, in the UK anyway, would have been Penguin Science Fiction, which also republished it.

As you've confirmed that this is the story, I'm answering just for the record. After some delay, because of problems with my Internet connection.

A short story (I think) from the 1950s/60s/70s:

Original publication year 1955, but the impressive list of republications shows a new appearance almost every year up to 1990, and a few after 2000 as well. Some of these are merely new editions, but even so, it keeps on and on coming back.

An expedition to a planet which is mainly water on the surface:

"Expedition" is perhaps an understatement: "The Colonial Team was a practical, hard-working outfit—two thousand people who'd been given twenty years to size up and tame down the brand-new world of Sutang to the point where a huundred thousand colonists could be settled on it, in reasonable safety and comfort."

The planet isn't just water. For example, the station manager Nirmond complains about the protagonist Cord keeping "unidentified and possibly deadly vermin in the woods back of the station". But most of the action does take place on water. On page 3, Nirmond drives Cord and others down to the edge of the bay in a treadcar, and they stop at the edge of a marshy cove, from which they transfer to a "raft". These are organisms that look like "lily pads twenty-five feet across". The raft they take is called Grandpa, a "regular monster" at fifty feet across.

One of the human visitors sees, below him in the water, something threatening like a yellow hammerhead shark:

"The reed bed to their right was thick with Yellowheads, a colony of them. Vaguely froggy things, man-sized and better. Of all the creatures he'd discovered in the bay, Cord liked them least. The flabby, sacklike bodies clung with four thin limbs to the upper sections of the twenty-foot reeds that lined the channel. They hardly ever moved, but their huge bulging eyes seemed to take in everything that went on about them. Every so often, a downy swamp bug came close enough; and a Yellowhead would open its vertical, enormous, tooth-lined slash of a mouth, extend the whole front of its face like a bellows in a flashing strike; and the bug would be gone. They might be useful, but Cord hated them."

Or, later on and with a different predator: "It was the shattering of his [Cord's] hopes to swim to shore from here. Fifty yards away, the creature from which the rubbery thing had been fleeing showed briefly on the surface, as it turned away from the raft; and that glance was all he needed. The ivory-white body and gaping jaws were similar enough to those of the sharks of Earth to indicate the pursuer's nature. The important difference was that wherever the White Hunters of the Zlanti Deep went, they went by the thousands."