[djbebs]

There is no such thing as international law.

Its not illegal border crossing either because there is de facto no border there.

A dejure argument doesn't make sense because no matter which side of the argument you take, crossing into crimea isn't on its own illegal. At worst you're supposed to get a visa, but Ukraine has a pretty open policy so that is generally not required for tourism under 90 days.

Presumably if you're coming from Russia you already have a visa to be in Russia too