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I'm designing an ocean planet. I did a quick search with this question on the interwebs, but nothing that came up was helpful (I don't have any physics or scientific background). I was wondering whether on an ocean planet, with a gravity of about three-fourths that of earth, with a radius comparable to earth, if the water would be any different (less dense, etc)?
Thanks!
Gravity most definitely affects the density of water. See Gliese 436b, for example. Gliese 436b is a water planet, who's intense pressure caused all water to turn to ice, but, thanks to greenhouse gasses, is on fire.
In chemistry, pressure can change states of matter similarly to heat. Intense pressure is the same as intense cold (or a lack of heat). However, it is the joint effort of termperature and pressure that decides the density of a substance, see the fact that we have water in all three states on Earth's surface even with gravity staying roughly the same. (This leads to a triple point, the temperature and pressure at which the liquid, solid, and gas forms of a substance exist at the same time which is super cool and I recommend you check it out if you're ever bored.)
But, to answer your question, the water on your world could be the same density, less dense, or more dense depending on the temperature. Here is a link to a water phase diagram: http://www1.lsbu.ac.uk/water/water_phase_diagram.html Depending on what density you want your planet's water to have, you can decide on a temperature, or vise versa.
Water is, generally speaking, incompressible. It doesn't act like other liquids, at least not until you treat is as though it's already at absurd compression as it is. I highly doubt that water would be functionally different.
Additionally, a fun fact pertaining to water: If it floats, it will float regardless of gravity. Gravity is a significant part of the equation for flotation, so barring other issues, if something floats on Earth, it'll float on a world with half gravity, or even double gravity. Of course, that's not all there is too it - A ship that's structurally sound in half gravity may not be structurally sound in double gravity, etc
Someone else posted a better answer to how gravity and density works in relation to water, so I decided to focus on a different way that gravity would affect your water: how your water would potentially act on your planet.
This question made me think of this blog post that mentions some stuff about how low gravity would affect water on the Moon.
Because the moon has low gravity, the water would be launched upward more easily, along with anything that was swimming in it. That means larger waves. However, it likely wouldn't be that much larger because your planet has only 3/4th the gravity of Earth compared to the Moon's 1/6th.
That's the only thing I could think of that would make the water different off the top of my head, I hope this helped!
