I know that thestrals pulled the carriages for the non-first year students, but am not sure whether any magical creature pulled the boats for the first year students at Hogwarts. So is a magical creature(s) pulling the boats across the lake?
The boats are most likely magical
The biggest piece of evidence that the boats move by magic is that Hagrid actually uses a spell to speed up the trip to shore after rescuing Harry from the island the Durleys were
holding him hostage staying on:
"Seems a shame ter row, though," said Hagrid, giving Harry another of his sideways looks. "If I was ter — er — speed things up a bit, would yeh mind not mentionin' it at Hogwarts?"
"Of course not," said Harry, eager to see more magic. Hagrid pulled out the pink umbrella again, tapped it twice on the side of the boat, and they sped off toward land.
Harry Potter and the Philosopher's Stone: Chapter 6 - "The Journey from Platform Nine and Three-Quarters"
We know Hagrid isn't supposed to be using magic, so the boats at Hogwarts would have been enchanted by a professor prior to the first-years' arrival.
So, knowing that there is in fact a spell that would propel boats forward, why complicate things by adding creatures? Creatures that would need to be trained, fed, housed, etc. Much simpler using magic I would say
The boats move all at the same time to the same direction, without any identification of sail, paddles or engine. It seems the boats move by magic itself:
The narrow path had opened suddenly onto the edge of a great black lake. Hagrid called, pointing to a fleet of little boats sitting in the water by the shore. "Everyone in?" shouted Hagrid, who had a boat to himself. "Right then -- FORWARD!" And the fleet of little boats moved off all at once, gliding across the lake, which was as smooth as glass. Everyone was silent, staring up at the great castle overhead. It towered over them as they sailed nearer and nearer to the cliff on which it stood. "Heads down!" yelled Hagrid as the first boats reached the cliff; they all bent their heads and the little boats carried them through a curtain of ivy that hid a wide opening in the cliff face. They were carried along a dark tunnel, which seemed to be taking them right underneath the castle, until they reached a kind of underground harbour, where they clambered out onto rocks and pebbles.
Harry Potter and the Philosopher's Stone - Chapter 6 - 'The Journey from Platform Nine and Three-Quarters'
Of course there could still be some invisible marine creature pulling the boat, but that would be just speculation. Do also note the lake is "as smooth as glass", which rules out a creature that swims up on the surface.