It doesn't necessarily need to be breathable by humans. But it should be room temperature (at which mercury is liquid). My concern is whether mercury oceans could remain stable or would they all get locked up in metallic compounds and precipitate?
You would have many problems: first of all, the abundance of mercury it's not that high to realistically be able to fill in oceans. In the solar system it is one of the rarest element, even less abundant than platinum.
Moreover, even assuming that you had that much mercury, you would stumble into another, serious issue: the density of mercury is 13.5 $g/cm^3$, while the density of basalt, the main component of oceanic bottom, is only 3 $g/cm^3$. This would mean that an ocean of mercury would quickly and dramatically sink below the crust.
You would want the planet's crust to be dense enough to prevent the mercury from going underground. However, denser materials are typically metallic, and would form amalgams with the mercury. However, there is a solution: Make the planet an giant ball of mercury large enough to have its own gravity, and possibly with islands floating on the surface.
If your goal is just to have a planet with liquid metal oceans, not specifically mercury, you could go with gallium. Its melting point is just ~30°C (~85°F), so a little warm but still perfectly survivable and even comfortable for humans. And it's much more common than mercury. Not sure about the feasibility of covering an entire planet in gallium seas, but smaller bodies of liquid gallium (ponds and lakes) might be possible.
Any life as we know it can't exist on the planet. Nor volcanoes. You can't have a sulfur cycle
In addition to L Dutch points, an ocean of mercury can not exist on any planet with a sulfur cycle. Any biological processes that produce hydrogen sulfide (sewer gas) or natural volcanic systems (sulfur dioxide) putting trace amounts of sulfur in the air will cause issues for your mercury sea.
Mercury and sulfur interact and form mercury sulfide, a solid that will tarnish your mercury sea like rust, forming a thin crust over the top, like a very thin ice.
This interaction is so rapid that you can buy mercury cleanup kits consisting of sulfur blocks that absorb the mercury.
In addition to the other considerations given here, you also have the problem of oxygen. It's very abundant and loves to make compounds with just about everything. All that liquid mercury would quickly (on a geological timescale, that is) become solid mercury oxide and mercury salts of oxyacids. If you can make it work, though, maybe you have mercury oxide mixed in with the sand on the beaches, giving it a pink or light orange color. That's optional because this is all pretty handwavy anyway.
Maybe some kind of weird plant life that eats mercury oxide and poops mercury could give you those seas. But then, like other answers say, you run into the problems of abundance and density.
You might prefer a sodium-potassium mixture. NaK is a liquid at room temperature, and both elements are far more abundant than mercury. Also it licks the density problem. You still need something to crack them from the oxides though. Oxygen is everywhere and it's thirsty.
Oh there's another problem: water. Water is also very abundant. In earthlike conditions it will be liquid and also make seas, which will float on top of the mercury. The NaK is better in that way because it's less dense than water, but it reacts like mad with water. In either case, you need to make sure there isn't much free liquid water on the surface of your planet. You can achieve that with conditions that crack the hydrogen from the oxygen, allowing the hydrogen to dissipate to space via Jeans escape, or by making the surface a bit cold. (There are probably other ways too.)
You can crack water with lightning or with my favorite method: more xenobiological handwaving.
The cold-planet method works because NaK is still liquid down to -12 C or so (at 1 atm of pressure). If conditions are right, you can have a water permafrost almost everywhere without it getting so cold as to freeze the NaK. The water ice will still react with the NaK, just more slowly.
However, down at the bottom of the NaK seas, the high pressure will make the NaK solid and the water underneath it liquid (unless it's really cold down there somehow, which I think is pretty much impossible). That complicates things. But I think it's safe enough to say a layer of NaK hydroxides will form, precluding any further reaction.
Or it's possible there was never water under the NaK, or not enough to matter. All the water is at the surface. Maybe your planet is tidally locked around a mediocre star, like an M0. The NaK seas are near high noon and the volatile water there has evaporated and snowed down near the terminator and on the dark side. NaK will still react with the rock underneath, but again I think a non-reactive intermediate layer saves you. Unless you have enough activity to bring magma to the surface. Then I don't think there's anything you can do.
Ah so this got pretty long, heh. Hope it helps.
It might work if the rest of the planet is made of gold.
But if the planet is made of lighter materials than mercury, the ocean will slowly but surely seep through towards the planet's core. Earth's oceans are only stable because the ocean bottom is heavier than water, and even so, lots of water seeps through the Earth's crust and the upper mantle contains lots of water.
Broken down alien liquid metal telescope
I think your main hurdle is a physically plausible explanation of how get so much mercury in one place. Despite its rarity in Earth's crust (0.05 mg/kg), mercury collects at relatively high concentrations of 25000 mg/kg in its ores. This is probably due to its unusual chemistry, which is mimicked by no other metal. However, I'm not sure I can come up with a good "natural" explanation how you get enough for an ocean and what could form the ocean floor on geological timescales (tungsten doesn't form amalgams and is denser than mercury, so it's a possible floor material).
The best idea I can come up with is that it was part of an alien megastructure. A liquid mirror telescope could be made by filling a crater with mercury. Or maybe it's a light concentrator for long distance communication. Or maybe it is some alien technology that we don't really understand, but large amounts of mercury were necessary. Anyway, the aliens left and the megastructure decayed (the walls of the crater collapsed or the containment vessel failed due to mercury embrittlement), flooding the planet with mercury.
I think the nearly irreconcilable problem you have constructing a mercury ocean world is explaining the process by which so much of that particular liquid element became concentrated on the world at the exclusion of others more likely to be present at similar temperatures. For example, based on the relative abundance of elements in the universe you would be about 10 times more likely to have oceans of rubidium and 100 times more likely to have oceans of Gallium. That's not even considering why liquid elements would be present at the exclusion of much more likely liquid compounds.
I say nearly irreconcilable because it would depend on the age of the universe of your world. As the universe ages heavier elements such as mercury will become more common relative to the other lower atomic mass elements due to stellar nucleosynthesis. As the universe approaches heat death, planets orbiting active stars (as I'm assuming yours does) may be composed mostly of elements heavier than iron, which would reduce the possibilities for liquid compounds. It seems this would make liquid elements the increasingly likely constituent of any oceans and so make mercury oceans more plausible.