Higher-than-usual tides are expected across the southern coast of England and the north of France due to a rare alignment of the sun and moon that happens only every 18 years.
The most spectacular scenes are expected at the picturesque French island of Mont Saint-Michel.
Perched on a rocky island topped with a Gothic Benedictine abbey, the Unesco World Heritage Site is exposed to some of Europe's strongest tides.
But the high tide due on Saturday evening is expected to be exceptional because of Friday's solar eclipse, with predictions that the water could reach as high as 14.15 metres.
On Friday police struggled to control a crowd of around 10,000 people drawn to what experts had described as "the highest tide of the century".
They were mainly disappointed because, despite a big enough surge of water to leave the only access road impassable, the waves fell a few inches short of expectations.
Officials at France's Navy Oceanic and Hydrological Service urged caution, warning that the high tide around 8pm GMT, "will come in faster than a running man" and would pose a danger to people venturing out too far.
In nearby Saint-Malo, crowds gathered by the beach to see waves lap at the town's stone walls.
Similar high waters are expected in parts of the UK's southern coast, where the Environment Agency moved to reassure people that the risk of flood remained "very low for the next few days".
A spokesman said: "High tides this weekend may lead to spray on promenades in some exposed coastal locations.
"We always monitor the flood risk situation closely, working alongside partners, including the Met Office and local authorities, and issue alerts and warnings if required."
The extreme spring tides were welcomed by surfers, however, as more than 100 headed to surf the River Severn's tidal bore.
Sky News Midlands correspondent Adele Robinson, in Minsterworth, Gloucestershire, said the bore is one of the largest in the world and waves reached 6ft 2in.
It is the second of three tidal bores expected this year, making 2015 a year of unusually-high tidal surges.
"It's essentially the incoming tide, which is pushing against the current of the river in the opposite direction, which creates waves that can travel a considerable distance."
The moon's gravitational pull and the rare alignment between the sun and the moon also have an effect on the rare high tide, she said.
One of those surfing the bore was Gary Wells, who managed to stay on his board for "about a minute-and-a-half", adding that it was "a bit trickier than you think."
