Can it [color=red]go on the land or is it just restricted to water?====> Go on the land or go on land here?


Either is acceptable there. It’s down to the speaker to decide (probably not in any conscious way) whether he sees the land as a specific piece of land or as land in a general sense.

I think I saw it on land a couple of times.====> What about this? Can it be “on the land” too?


“Can it go on the land or is it just restricted to water?”

My additional question.
Should the inversion of the verb be used in the second clause or not?
I don’t think it should.


Inversion (question word order) is necessary in the second clause. You have two independent questions joined by the word ‘or’ here. The is no ‘embedded’ question in that sentence.

[size=75]“The sensitivity of men to small matters, and their indifference to great ones, indicates a strange inversion.” ~ Blaise Pascal[/size]