[Hannah LS (OP)]

Muhammed asks:

Are there parts of the ocean that are continuous but have a different "sea level"? I know gravity is slightly different in different parts of the world as has been recorded on land, but is this phenomenon significant at sea?

Could you be in one place at sea that has a height at sea level of 0 and is used as a reference height and then travel a good distance away to another place where gravity is higher and thus sea level would actually be negative with respect to the original measurement point?

What do you think?

[Colin2B]

If you were to measure from the centre of the earth, then what you say would be true. However, it is very useful to have a sea level that doesn’t vary with gravity and this is defined as the geoid. This is also extended onto land and is very useful eg if you have a canal which follows the geoid height (or is a fixed ht above or below it) then water will not flow from one end to the other.
The physical shape of the earth is called the ellipsoid and is the one used by gps. The GRS80 ellipsoid and the geoid vary by about +85 meters west of Ireland to about -106 meters  in the south of India near Ceylon.

[Janus]

Just one addendum to what Colin2B said.  The geoid follows the surface of equal gravitational potential, not the surface of equal gravitational force.  So the local strength of gravity will vary at different points along the geoid, or at "sea level".

[evan_au]

Footnote: This discussion is about "mean sea level". Superimposed on the geoid are wind waves and tides which are ignored when you take a long-term average.

This discussion also ignores things like the antarctic ice shelf, where the surface of the liquid water is pushed down below the geoid by the kilometer of ice floating above it.

[Colin2B]

What @Janus and @evan_au have added is the reason I love the geoid, there is so much richness in it that it’s hard to know how much to tell a newcommer.
So, to whet Muhammed‘s appetite even further: where the geoid curves up or down a plumbline will not point geographically straight down towards the centre of the earth, but will lie perpendicular to the tangent to the geoid. Very good for detecting deposits of heavy minerals below the surface, along with measures of gravitational potential.

[DrMortimer]

I believe the gravity can't influence on water in such way

[Colin2B]

I believe the gravity can't influence on water in such way

In such a way as what?
Which of the answers are you suggesting can’t be true?