Is it possible to conjure a magic teleportation circle, where one point is deep in the ocean, and the other point is wherever I want (like in front of my enemies)?
No, this idea doesn't hold water
Teleportation circle says:
Any creature that enters the portal instantly appears within 5 feet of the destination circle or in the nearest unoccupied space if that space is occupied.
The ocean is not a creature and thus the water would not be teleported through to the other side.1
Another serious problem with this idea is that the spell only links to permanent teleportation circles. You can't just choose an arbitrary point to teleport to.
As you cast the spell, you draw a 10-foot-diameter circle on the ground inscribed with sigils that link your location to a permanent teleportation circle of your choice whose sigil sequence you know and that is on the same plane of existence as you.
So unless your enemies happened to be standing right in front of a permanent teleportation circle whose sigil sequence you know you would be out of luck anyways without some very serious and impractical planning.
It's also worth noting that the spell must be cast on the ground so you can't just cast it "in the ocean" in an arbitrary location. It must be somewhere with ground (such as the ocean floor).
If you have access to the 9th level gate spell this might work since it does not have a restriction on what it teleports.2 However, you do have to be able to see where you are casting the gate and also the destination must be on another plane. As a side note, it will also cost 5,000gp. At that point you might be better off just using wish to put large amounts of water on top of your enemy if it was that important.
Arcane gate would be a great option (probably the best option) if not for the restriction on what can pass through:
Any creature or object entering the portal exits from the other portal as if the two were adjacent to each other; passing through a portal from the nonportal side has no effect.
As fluids are generally held to not be considered objects, RAW would say using the spell in this way would not work.
1 - However if you really work at it you might be able to teleport a few sea creatures. Anybody up for a fish fry?
2 - thanks @goodguy5
No, you can't use Teleportation Circle in that manner. Teleportation Circle can only let creatures through and is terribly inconvenient with its requirement that the destination circle has to exist, see Rubiksmoose's answer.
However, if your objective is just to make a mess, there are other spells that can achieve this:
The 1st-level spell Create or Destroy Water can make rain in a 30-ft cube and extinguishes any open flames.
For a grander, more violent effect, Tsunami is the spell you want. However, it is an 8th-level spell.
Well, seeing as it requires a drawn circle of chalk and ink to do I am inclined to say no. Seems you would need it to not be washed away. Even if you say something like a spellcaster's focus to cast it, I will still say the circle would be made of the same material only you didn't require them on you.
TLDR: No, because there is no way to create the circle there with all the problems stated by other people above.