I think that the water would also be lifted

That makes sense.  On another note though, this spell is devastating in water combat if a ship is attacking you. I learned most ships don't do well when dropped from 100' 

Does anyone know if creatures that are Immune to spells & Magic Effects are affected by this spell?

Quote from ElfBoyPorter >>

What would happen if a creature used their Legendary Resistance to make the Dexterity Save? They are still in a field of ongoing reverse gravity.

In this scenario there are no fixed objects for them to grab onto. Weird question I know but my character has this spell and the likelyhood of encountering Legendary creatures right now are very high.

There is no ongoing reversal of gravity in the area, the spell description does not mention any creature entering the area on subsequent turns is affected by it, so no ongoing in the area just on the initially affected objects and creatures.  The spell also doesn't say that "If there are no fixed objects in the area no save is allowed!"

So It would really come down to what flavor the Dungeon Master wanted to add.  With Legendary resistance, maybe they just shrug it off, or dig their claws into the ground.  Even if there was not a tree to bush or other anchored object to grab onto even without legendary resistance you might shrug it off, dig their weapon into the ground or wall or a dead body.  

Personally though I would say anyone affected by it is at disadvantage on all attack rolls since they are "oscillating"  which indicates they are not fixed in place.  The Spell really should restrain them.

Do you believe in gravity?

nice imagine having a wizard or socerer just killing the intire party with reverse gravati

So does the creature have a speed of 0?

something something Australia joke

I'd say yes. Hang that fishy bastard out to dry!

dope spell

Agree on no ongoing reversal of gravity- only creatures in the area when the spell is cast are effected 

Disagree on the saving throw IMO if you can't reach a fixed object then there's no saving throw - regardless of legendary resistance 

I wonder if "oscillating slightly" leaves it open to interpret that any creatures at the top of the area are technically prone.

How much damage does it do though?

While concentrating on this spell, can you reverse the gravity, then turn it back to normal, then reverse it again?

Now we just need a spell that turns things inside out

No, Mending is another spell that I know of that requires a lodestone as part of its material components.

Quote from ProphetZarquon >>

Is Disintegrate the only other spell that calls for a lodestone? 

 Replying to this, no, Mending is another spell with the same material prerequisite.

What happens when you cast Reverse Gravity underwater?

Let's say you drop concentration right as all of the affected units reach the 100 foot mark. Would they be flung even higher into the air due to momentum and cause potentially even more fall damage?

The ship would have to be less than 50 feet long correct?