It seems to be that the Orb refrains from freezing the liquid if it is cast underwater.
The intention seems to be that throwing the orb underwater wouldn't be considered striking the water. Striking usually refers to as one thing coming into contact with another, with the intent to hit, and when your orb 'strikes the body of water' isn't clear. It is also IN a body of water, so the orb isn't really capable of striking a body of water if it is already in one.
If the globe strikes a body of water or a liquid that is principally water (not including water-based creatures), it freezes the liquid to a depth of 6 inches over an area 30 feet square
Emphasis mine.
While getting into a bit of semantics depth is the distance from the top of something to the bottom. It would be slightly unintuitive if one could consider something 'the top of' if it started in the middle of something, such as the ocean or body of water you are in. Depth doesn't seem to apply properly in this scenario.
If it froze underwater using area and depth would be an inconvenient way to do things.
If the intention of the spell was to properly freeze while cast underwater it would, for more ease and out of a normal expectation, probably freeze in a radius or a more spherical shape. The wording of the spell seems to indicate it is made to freeze the surface of water. It is measured in area and with depth included, not a measurement and description you would use for underwater (at least not normally).
With this and the phrasing of 'strike' and with the observation that the area the spell freezes is an 'area' (with a six inch depth) it can be argued that the orb would not instantly explode with contact with water, if it were already underwater. But further on...
It would not freeze any water.
The freezing of the water is not directly related to the cold damage the spell does. With the orb not properly preforming it's second function, the freezing of water, there is no reason that the freezing would take place. Cold damage does not freeze water without DM intervention (or a certain effect says so, like with the case of some spells like Wall of Water).
Lastly...
There is no difference between casting the spell or taking the orb and throwing it.
It shatters on impact, with the same effect as the normal casting of the spell. You can also set the globe down without shattering it. After 1 minute, if the globe hasn’t already shattered, it explodes.
Emphasis mine.
Outside of a slight word difference between 'explodes' and 'shatters' the spell functions the same whether you are throwing it or casting it and throwing it instantly as apart of the spell. There is no difference between the orb shattering or exploding in a body of water. As long as the orb doesn't strike the water it shouldn't be.
Perhaps more is needed....
If designer intent was that the spell was to instantly freeze water it hit (or started in), excluding water based creatures such as elementals, then it could be that this interpretation is incorrect. I've tweeted Crawford over the issue to see what the intent is.
You can teleport or plane shift out of Otiluke's resilient sphere
Provided that the caster is the one inside the sphere teleport will work. Teleport has a range of 10ft so it's area of effect will be limited by the bounds of the sphere so the caster and any targets must be inside. Teleport says:
This spell instantly transports you [...] to a destination you select.
This effect is not a movement effect, you aren't passing through the sphere. You instantly appear somewhere else.
Similarly Plane Shift has a range of touch and does not require a spell effect to travel through the sphere. It says:
You [..] are transported to a different place of existence.
Traveling to another plane does not require moving through the sphere. So you can Plane Shift out of the sphere. Misty Step, Thunder Step and any other teleportation effects will all work.
Interestingly Etherealness will even let you pass through the sphere itself as you are no longer on the same plane of existance though you can percieve it. We know this because Etherealness says:
You ignore all objects and effects that aren't on the Ethereal plane, allowing you to move through objects you perceive on the plane you originated from.
As you suggest spells for which this doesn't work specifically say so, as per Force Cage's description:
If the creature tires to use teleportation or interplaner travel to leave the cage, it must first make a Charisma saving throw. On a success, the creature can use that magic to exit the cage. On a failure, the creature can't exit the cage and wastes the use of the spell of effect. The cage also extends into the Ethereal Plane, blocking ethereal travel.
Otiluke's resilient sphere does not contain similar text and spell's only do what they say they do. Therefore the sphere does not prevent teleportation of interplanar travel.
Best Answer
Depends whether your DM considers the small globe created by Freezing Sphere to be an object, and, more generally, how open they are to their campaign embracing mutually assured destruction.
Some spells/abilities specifically call out created things as objects, others don't - referred to as 'item' or specific description of whatever it is 'the XYZ'. The rules can get a bit weird if nothing that isn't specifically called out as an object isn't counted as one, and many people are going to think that doesn't make sense (there is also no rule stating this - see below). Most DMs will rule that you can for example pick up the small globe with Mage Hand or try to launch it through an arrow slit with Catapult.
This is basically what determines if something is an object, although objects are alluded to in a lot of other vaguely defining senses. Under this, a small globe that you (or anyone else) can pick up and throw almost certainly counts. For extra bonus points, a crystal globe (an object) is a material component of the spell and nearly anyone would assume that the spell is referring to that globe (or a very similar one) in its description.
However, this situation is ripe for the most common source of the banhammer in 5e games - 'interpreting the rules'. Across forums and from some games I've sat in on (I would not play a game like this) it seems to be very common advice or practice to interpret the often loosely-worded 5e rules to deter optimization considered beyond the pale. Given a vague rule buried in the DMG and an easy loophole, any DM who wishes to ban this practice can do so without creating a houserule (considered very bad for some reason, especially in groups without anyone who has ever played DnD until recently) by deciding the globe is a 'spell effect' and not 'an inanimate object such as a small globe'.
There are a lot of ways an 11th level Wizard can break 5e DnD and destroy his, her or its foes from complete and utter safety if given time. Freezing Sphere is not even one of the more efficient methods, unless your foes are groups of low hp enemies conveniently clustered for maximum effect (the local college who poo poo'd your research paper, for example). It is a relatively straightforward method that doesn't rely on moving parts, allies, dice rolls or mind control though so it has that going for it.
Regardless, if like many DMs the DM at whichever table this is doesn't want to consider such things as Magically Assured Destruction or secret cross-national archmagi pacts against long-distance magical weaponry, they likely won't want the player characters to be doing this. The NPCs won't because the DM doesn't want them to, and the PCs won't because the DM will say 'no' (or find some convoluted, theoretically-RAW way to say no).
So, can Freezing Spheres be teleported? Yeah, probably. Even if some guy on twitter says otherwise, you can just glue it to a squirrel and teleport that at just the right moment. Or whatever, there are workarounds.
But whether you can teleport a Freezing Sphere in your game depends on the DM and the social contract at your table. Because that kind of game - about brinksmanship, inter-university politics and interplanar early warning systems - is not necessarily the kind of game that everyone sat down and agreed to play.